api/msgpack-rpc: Improve error infrastructure

- Add error type information to `Error`
- Rename `set_api_error` to `api_set_error` for consistency with other api_*
  functions/macros.
- Refactor the api_set_error macro to accept formatted strings and error types
- Improve error messages
- Wrap error messages with gettext macro
- Refactor msgpack-rpc serialization to transform Error instances into [type,
  message] arrays
- Add error type information to API metadata
- Normalize nvim->client and client->nvim error handling(change
  channel_send_call to accept an Error pointer instead of the `errored` boolean
  pointer)
- Use macro to initialize Error structures

11 files changed, 201 insertions, 146 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 8355bfe868..62559163d8 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -125,7 +125,7 @@ ArrayOf(String) buffer_get_slice(Buffer buffer,
     int64_t lnum = start + (int64_t)i;
 
     if (lnum > LONG_MAX) {
-      set_api_error("Line index is too high", err);
+      api_set_error(err, Validation, _("Line index is too high"));
       goto end;
     }
 
@@ -175,7 +175,9 @@ void buffer_set_slice(Buffer buffer,
   end = normalize_index(buf, end) + (include_end ? 1 : 0);
 
   if (start > end) {
-    set_api_error("start > end", err);
+    api_set_error(err,
+                  Validation,
+                  _("Argument \"start\" is higher than \"end\""));
     return;
   }
 
@@ -189,7 +191,9 @@ void buffer_set_slice(Buffer buffer,
 
   for (size_t i = 0; i < new_len; i++) {
     if (replacement.items[i].type != kObjectTypeString) {
-      set_api_error("all items in the replacement array must be strings", err);
+      api_set_error(err,
+                    Validation,
+                    _("All items in the replacement array must be strings"));
       goto end;
     }
 
@@ -201,7 +205,7 @@ void buffer_set_slice(Buffer buffer,
   switch_to_win_for_buf(buf, &save_curwin, &save_curtab, &save_curbuf);
 
   if (u_save((linenr_T)(start - 1), (linenr_T)end) == FAIL) {
-    set_api_error("Cannot save undo information", err);
+    api_set_error(err, Exception, _("Failed to save undo information"));
     goto end;
   }
 
@@ -211,7 +215,7 @@ void buffer_set_slice(Buffer buffer,
   size_t to_delete = (new_len < old_len) ? (size_t)(old_len - new_len) : 0;
   for (size_t i = 0; i < to_delete; i++) {
     if (ml_delete((linenr_T)start, false) == FAIL) {
-      set_api_error("Cannot delete line", err);
+      api_set_error(err, Exception, _("Failed to delete line"));
       goto end;
     }
   }
@@ -228,12 +232,12 @@ void buffer_set_slice(Buffer buffer,
     int64_t lnum = start + (int64_t)i;
 
     if (lnum > LONG_MAX) {
-      set_api_error("Index value is too high", err);
+      api_set_error(err, Validation, _("Index value is too high"));
       goto end;
     }
 
     if (ml_replace((linenr_T)lnum, (char_u *)lines[i], false) == FAIL) {
-      set_api_error("Cannot replace line", err);
+      api_set_error(err, Exception, _("Failed to replace line"));
       goto end;
     }
     // Mark lines that haven't been passed to the buffer as they need
@@ -246,12 +250,12 @@ void buffer_set_slice(Buffer buffer,
     int64_t lnum = start + (int64_t)i - 1;
 
     if (lnum > LONG_MAX) {
-      set_api_error("Index value is too high", err);
+      api_set_error(err, Validation, _("Index value is too high"));
       goto end;
     }
 
     if (ml_append((linenr_T)lnum, (char_u *)lines[i], 0, false) == FAIL) {
-      set_api_error("Cannot insert line", err);
+      api_set_error(err, Exception, _("Failed to insert line"));
       goto end;
     }
 
@@ -415,7 +419,7 @@ void buffer_set_name(Buffer buffer, String name, Error *err)
   }
 
   if (ren_ret == FAIL) {
-    set_api_error("failed to rename buffer", err);
+    api_set_error(err, Exception, _("Failed to rename buffer"));
   }
 }
 
@@ -425,7 +429,7 @@ void buffer_set_name(Buffer buffer, String name, Error *err)
 /// @return true if the buffer is valid, false otherwise
 Boolean buffer_is_valid(Buffer buffer)
 {
-  Error stub = {.set = false};
+  Error stub = ERROR_INIT;
   return find_buffer_by_handle(buffer, &stub) != NULL;
 }
 
@@ -460,7 +464,7 @@ ArrayOf(Integer, 2) buffer_get_mark(Buffer buffer, String name, Error *err)
   }
 
   if (name.size != 1) {
-    set_api_error("mark name must be a single character", err);
+    api_set_error(err, Validation, _("Mark name must be a single character"));
     return rv;
   }
 
@@ -478,7 +482,7 @@ ArrayOf(Integer, 2) buffer_get_mark(Buffer buffer, String name, Error *err)
   }
 
   if (posp == NULL) {
-    set_api_error("invalid mark name", err);
+    api_set_error(err, Validation, _("Invalid mark name"));
     return rv;
   }
 
diff --git a/src/nvim/api/private/defs.h b/src/nvim/api/private/defs.h
index 2dd229ec5f..76ac23a521 100644
--- a/src/nvim/api/private/defs.h
+++ b/src/nvim/api/private/defs.h
@@ -8,6 +8,7 @@
 #define ARRAY_DICT_INIT {.size = 0, .capacity = 0, .items = NULL}
 #define STRING_INIT {.data = NULL, .size = 0}
 #define OBJECT_INIT { .type = kObjectTypeNil }
+#define ERROR_INIT { .set = false }
 #define REMOTE_TYPE(type) typedef uint64_t type
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -16,8 +17,14 @@
 #endif
 
 // Basic types
+typedef enum {
+  kErrorTypeException,
+  kErrorTypeValidation
+} ErrorType;
+
 typedef struct {
-  char msg[256];
+  ErrorType type;
+  char msg[1024];
   bool set;
 } Error;
 
diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index 14a820aa1b..093cb0e55f 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -48,7 +48,7 @@ bool try_end(Error *err)
       discard_current_exception();
     }
 
-    set_api_error("Keyboard interrupt", err);
+    api_set_error(err, Exception, _("Keyboard interrupt"));
     got_int = false;
   } else if (msg_list != NULL && *msg_list != NULL) {
     int should_free;
@@ -64,7 +64,7 @@ bool try_end(Error *err)
       free(msg);
     }
   } else if (did_throw) {
-    set_api_error((char *)current_exception->value, err);
+    api_set_error(err, Exception, "%s", current_exception->value);
   }
 
   return err->set;
@@ -80,7 +80,7 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
   hashitem_T *hi = hash_find(&dict->dv_hashtab, (uint8_t *) key.data);
 
   if (HASHITEM_EMPTY(hi)) {
-    set_api_error("Key not found", err);
+    api_set_error(err, Validation, _("Key not found"));
     return (Object) OBJECT_INIT;
   }
 
@@ -101,17 +101,17 @@ Object dict_set_value(dict_T *dict, String key, Object value, Error *err)
   Object rv = OBJECT_INIT;
 
   if (dict->dv_lock) {
-    set_api_error("Dictionary is locked", err);
+    api_set_error(err, Exception, _("Dictionary is locked"));
     return rv;
   }
 
   if (key.size == 0) {
-    set_api_error("Empty dictionary keys aren't allowed", err);
+    api_set_error(err, Validation, _("Empty dictionary keys aren't allowed"));
     return rv;
   }
 
   if (key.size > INT_MAX) {
-    set_api_error("Key length is too high", err);
+    api_set_error(err, Validation, _("Key length is too high"));
     return rv;
   }
 
@@ -121,7 +121,7 @@ Object dict_set_value(dict_T *dict, String key, Object value, Error *err)
     // Delete the key
     if (di == NULL) {
       // Doesn't exist, fail
-      set_api_error("Key doesn't exist", err);
+      api_set_error(err, Validation, _("Key \"%s\" doesn't exist"), key.data);
     } else {
       // Return the old value
       rv = vim_to_object(&di->di_tv);
@@ -170,7 +170,7 @@ Object get_option_from(void *from, int type, String name, Error *err)
   Object rv = OBJECT_INIT;
 
   if (name.size == 0) {
-    set_api_error("Empty option name", err);
+    api_set_error(err, Validation, _("Empty option name"));
     return rv;
   }
 
@@ -181,7 +181,10 @@ Object get_option_from(void *from, int type, String name, Error *err)
                                       type, from);
 
   if (!flags) {
-    set_api_error("invalid option name", err);
+    api_set_error(err,
+                  Validation,
+                  _("Invalid option name \"%s\""),
+                  name.data);
     return rv;
   }
 
@@ -197,10 +200,16 @@ Object get_option_from(void *from, int type, String name, Error *err)
       rv.data.string.data = stringval;
       rv.data.string.size = strlen(stringval);
     } else {
-      set_api_error(N_("Unable to get option value"), err);
+      api_set_error(err,
+                    Exception,
+                    _("Unable to get value for option \"%s\""),
+                    name.data);
     }
   } else {
-    set_api_error(N_("internal error: unknown option type"), err);
+    api_set_error(err,
+                  Exception,
+                  _("Unknown type for option \"%s\""),
+                  name.data);
   }
 
   return rv;
@@ -216,24 +225,33 @@ Object get_option_from(void *from, int type, String name, Error *err)
 void set_option_to(void *to, int type, String name, Object value, Error *err)
 {
   if (name.size == 0) {
-    set_api_error("Empty option name", err);
+    api_set_error(err, Validation, _("Empty option name"));
     return;
   }
 
   int flags = get_option_value_strict(name.data, NULL, NULL, type, to);
 
   if (flags == 0) {
-    set_api_error("invalid option name", err);
+    api_set_error(err,
+                  Validation,
+                  _("Invalid option name \"%s\""),
+                  name.data);
     return;
   }
 
   if (value.type == kObjectTypeNil) {
     if (type == SREQ_GLOBAL) {
-      set_api_error("unable to unset option", err);
+      api_set_error(err,
+                    Exception,
+                    _("Unable to unset option \"%s\""),
+                    name.data);
       return;
     } else if (!(flags & SOPT_GLOBAL)) {
-      set_api_error("cannot unset option that doesn't have a global value",
-                     err);
+      api_set_error(err,
+                    Exception,
+                    _("Cannot unset option \"%s\" "
+                      "because it doesn't have a global value"),
+                    name.data);
       return;
     } else {
       unset_global_local_option(name.data, to);
@@ -245,7 +263,10 @@ void set_option_to(void *to, int type, String name, Object value, Error *err)
 
   if (flags & SOPT_BOOL) {
     if (value.type != kObjectTypeBoolean) {
-      set_api_error("option requires a boolean value", err);
+      api_set_error(err,
+                    Validation,
+                    _("Option \"%s\" requires a boolean value"),
+                    name.data);
       return;
     }
 
@@ -253,12 +274,18 @@ void set_option_to(void *to, int type, String name, Object value, Error *err)
     set_option_value_for(name.data, val, NULL, opt_flags, type, to, err);
   } else if (flags & SOPT_NUM) {
     if (value.type != kObjectTypeInteger) {
-      set_api_error("option requires an integer value", err);
+      api_set_error(err,
+                    Validation,
+                    _("Option \"%s\" requires an integer value"),
+                    name.data);
       return;
     }
 
     if (value.data.integer > INT_MAX || value.data.integer < INT_MIN) {
-      set_api_error("Option value outside range", err);
+      api_set_error(err,
+                    Validation,
+                    _("Value for option \"%s\" is outside range"),
+                    name.data);
       return;
     }
 
@@ -266,7 +293,10 @@ void set_option_to(void *to, int type, String name, Object value, Error *err)
     set_option_value_for(name.data, val, NULL, opt_flags, type, to, err);
   } else {
     if (value.type != kObjectTypeString) {
-      set_api_error("option requires a string value", err);
+      api_set_error(err,
+                    Validation,
+                    _("Option \"%s\" requires a string value"),
+                    name.data);
       return;
     }
 
@@ -296,7 +326,7 @@ buf_T *find_buffer_by_handle(Buffer buffer, Error *err)
   buf_T *rv = handle_get_buffer(buffer);
 
   if (!rv) {
-    set_api_error("Invalid buffer id", err);
+    api_set_error(err, Validation, _("Invalid buffer id"));
   }
 
   return rv;
@@ -307,7 +337,7 @@ win_T * find_window_by_handle(Window window, Error *err)
   win_T *rv = handle_get_window(window);
 
   if (!rv) {
-    set_api_error("Invalid window id", err);
+    api_set_error(err, Validation, _("Invalid window id"));
   }
 
   return rv;
@@ -318,7 +348,7 @@ tabpage_T * find_tab_by_handle(Tabpage tabpage, Error *err)
   tabpage_T *rv = handle_get_tabpage(tabpage);
 
   if (!rv) {
-    set_api_error("Invalid tabpage id", err);
+    api_set_error(err, Validation, _("Invalid tabpage id"));
   }
 
   return rv;
@@ -376,7 +406,7 @@ bool object_to_vim(Object obj, typval_T *tv, Error *err)
 
     case kObjectTypeInteger:
       if (obj.data.integer > INT_MAX || obj.data.integer < INT_MIN) {
-        set_api_error("Integer value outside range", err);
+        api_set_error(err, Validation, _("Integer value outside range"));
         return false;
       }
 
@@ -424,7 +454,9 @@ bool object_to_vim(Object obj, typval_T *tv, Error *err)
         String key = item.key;
 
         if (key.size == 0) {
-          set_api_error("Empty dictionary keys aren't allowed", err);
+          api_set_error(err,
+                        Validation,
+                        _("Empty dictionary keys aren't allowed"));
           // cleanup
           dict_free(tv->vval.v_dict, true);
           return false;
@@ -513,6 +545,7 @@ Dictionary api_metadata(void)
 
   if (!metadata.size) {
     msgpack_rpc_init_function_metadata(&metadata);
+    init_error_type_metadata(&metadata);
     init_type_metadata(&metadata);
     provider_init_feature_metadata(&metadata);
   }
@@ -520,6 +553,21 @@ Dictionary api_metadata(void)
   return copy_object(DICTIONARY_OBJ(metadata)).data.dictionary;
 }
 
+static void init_error_type_metadata(Dictionary *metadata)
+{
+  Dictionary types = ARRAY_DICT_INIT;
+
+  Dictionary exception_metadata = ARRAY_DICT_INIT;
+  PUT(exception_metadata, "id", INTEGER_OBJ(kErrorTypeException));
+
+  Dictionary validation_metadata = ARRAY_DICT_INIT;
+  PUT(validation_metadata, "id", INTEGER_OBJ(kErrorTypeValidation));
+
+  PUT(types, "Exception", DICTIONARY_OBJ(exception_metadata));
+  PUT(types, "Validation", DICTIONARY_OBJ(validation_metadata));
+
+  PUT(*metadata, "error_types", DICTIONARY_OBJ(types));
+}
 static void init_type_metadata(Dictionary *metadata)
 {
   Dictionary types = ARRAY_DICT_INIT;
@@ -704,7 +752,9 @@ static void set_option_value_for(char *key,
         if (try_end(err)) {
           return;
         }
-        set_api_error("problem while switching windows", err);
+        api_set_error(err,
+                      Exception,
+                      _("Problem while switching windows"));
         return;
       }
       set_option_value_err(key, numval, stringval, opt_flags, err);
@@ -745,6 +795,6 @@ static void set_option_value_err(char *key,
       return;
     }
 
-    set_api_error(errmsg, err);
+    api_set_error(err, Exception, "%s", errmsg);
   }
 }
diff --git a/src/nvim/api/private/helpers.h b/src/nvim/api/private/helpers.h
index f3ecdaacc4..f29deb53f9 100644
--- a/src/nvim/api/private/helpers.h
+++ b/src/nvim/api/private/helpers.h
@@ -8,10 +8,13 @@
 #include "nvim/memory.h"
 #include "nvim/lib/kvec.h"
 
-#define set_api_error(message, err)                \
-  do {                                             \
-    xstrlcpy(err->msg, message, sizeof(err->msg)); \
-    err->set = true;                               \
+#define api_set_error(err, errtype, ...)             \
+  do {                                               \
+    snprintf((err)->msg,                             \
+             sizeof((err)->msg),                     \
+             __VA_ARGS__);                           \
+    (err)->set = true;                               \
+    (err)->type = kErrorType##errtype;               \
   } while (0)
 
 #define OBJECT_OBJ(o) o
diff --git a/src/nvim/api/tabpage.c b/src/nvim/api/tabpage.c
index 91020d6850..72168d936a 100644
--- a/src/nvim/api/tabpage.c
+++ b/src/nvim/api/tabpage.c
@@ -116,7 +116,7 @@ Window tabpage_get_window(Tabpage tabpage, Error *err)
 /// @return true if the tab page is valid, false otherwise
 Boolean tabpage_is_valid(Tabpage tabpage)
 {
-  Error stub = {.set = false};
+  Error stub = ERROR_INIT;
   return find_tab_by_handle(tabpage, &stub) != NULL;
 }
 
diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 43f2aafdc8..93aa0439f0 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -1,5 +1,6 @@
 #include <assert.h>
 #include <stdint.h>
+#include <inttypes.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
@@ -117,7 +118,7 @@ Object vim_eval(String str, Error *err)
   typval_T *expr_result = eval_expr((char_u *) str.data, NULL);
 
   if (!expr_result) {
-    set_api_error("Failed to eval expression", err);
+    api_set_error(err, Exception, _("Failed to evaluate expression"));
   }
 
   if (!try_end(err)) {
@@ -139,7 +140,7 @@ Object vim_eval(String str, Error *err)
 Integer vim_strwidth(String str, Error *err)
 {
   if (str.size > INT_MAX) {
-    set_api_error("String length is too high", err);
+    api_set_error(err, Validation, _("String length is too high"));
     return 0;
   }
 
@@ -194,7 +195,7 @@ ArrayOf(String) vim_list_runtime_paths(void)
 void vim_change_directory(String dir, Error *err)
 {
   if (dir.size >= MAXPATHL) {
-    set_api_error("directory string is too long", err);
+    api_set_error(err, Validation, _("Directory string is too long"));
     return;
   }
 
@@ -206,7 +207,7 @@ void vim_change_directory(String dir, Error *err)
 
   if (vim_chdir((char_u *)string)) {
     if (!try_end(err)) {
-      set_api_error("failed to change directory", err);
+      api_set_error(err, Exception, _("Failed to change directory"));
     }
     return;
   }
@@ -364,18 +365,13 @@ void vim_set_current_buffer(Buffer buffer, Error *err)
   }
 
   try_start();
-  if (do_buffer(DOBUF_GOTO, DOBUF_FIRST, FORWARD, buf->b_fnum, 0) == FAIL) {
-    if (try_end(err)) {
-      return;
-    }
-
-    char msg[256];
-    snprintf(msg, sizeof(msg), "failed to switch to buffer %d", (int)buffer);
-    set_api_error(msg, err);
-    return;
+  int result = do_buffer(DOBUF_GOTO, DOBUF_FIRST, FORWARD, buf->b_fnum, 0);
+  if (!try_end(err) && result == FAIL) {
+    api_set_error(err,
+                  Exception,
+                  _("Failed to switch to buffer %" PRIu64),
+                  buffer);
   }
-
-  try_end(err);
 }
 
 /// Gets the current list of window handles
@@ -422,16 +418,12 @@ void vim_set_current_window(Window window, Error *err)
 
   try_start();
   goto_tabpage_win(win_find_tabpage(win), win);
-
-  if (win != curwin) {
-    if (try_end(err)) {
-      return;
-    }
-    set_api_error("did not switch to the specified window", err);
-    return;
+  if (!try_end(err) && win != curwin) {
+    api_set_error(err,
+                  Exception,
+                  _("Failed to switch to window %" PRIu64),
+                  window);
   }
-
-  try_end(err);
 }
 
 /// Gets the current list of tabpage handles
@@ -481,7 +473,12 @@ void vim_set_current_tabpage(Tabpage tabpage, Error *err)
 
   try_start();
   goto_tabpage_tp(tp, true, true);
-  try_end(err);
+  if (!try_end(err) && tp != curtab) {
+    api_set_error(err,
+                  Exception,
+                  _("Failed to switch to tabpage %" PRIu64),
+                  tabpage);
+  }
 }
 
 /// Subscribes to event broadcasts
@@ -524,7 +521,7 @@ void vim_register_provider(uint64_t channel_id, String feature, Error *err)
   xstrlcpy(buf, feature.data, sizeof(buf));
 
   if (!provider_register(buf, channel_id)) {
-    set_api_error("Feature doesn't exist", err);
+    api_set_error(err, Validation, _("Feature doesn't exist"));
   }
 }
 
diff --git a/src/nvim/api/window.c b/src/nvim/api/window.c
index dd256f2b6d..751518424b 100644
--- a/src/nvim/api/window.c
+++ b/src/nvim/api/window.c
@@ -57,7 +57,9 @@ void window_set_cursor(Window window, ArrayOf(Integer, 2) pos, Error *err)
 
   if (pos.size != 2 || pos.items[0].type != kObjectTypeInteger ||
       pos.items[1].type != kObjectTypeInteger) {
-    set_api_error("\"pos\" argument must be a [row, col] array", err);
+    api_set_error(err,
+                  Validation,
+                  _("Argument \"pos\" must be a [row, col] array"));
     return;
   }
 
@@ -69,12 +71,12 @@ void window_set_cursor(Window window, ArrayOf(Integer, 2) pos, Error *err)
   int64_t col = pos.items[1].data.integer;
 
   if (row <= 0 || row > win->w_buffer->b_ml.ml_line_count) {
-    set_api_error("cursor position outside buffer", err);
+    api_set_error(err, Validation, _("Cursor position outside buffer"));
     return;
   }
 
   if (col > MAXCOL || col < 0) {
-    set_api_error("Column value outside range", err);
+    api_set_error(err, Validation, _("Column value outside range"));
     return;
   }
 
@@ -117,7 +119,7 @@ void window_set_height(Window window, Integer height, Error *err)
   }
 
   if (height > INT_MAX || height < INT_MIN) {
-    set_api_error("Height value outside range", err);
+    api_set_error(err, Validation, _("Height value outside range"));
     return;
   }
 
@@ -160,7 +162,7 @@ void window_set_width(Window window, Integer width, Error *err)
   }
 
   if (width > INT_MAX || width < INT_MIN) {
-    set_api_error("Width value outside range", err);
+    api_set_error(err, Validation, _("Width value outside range"));
     return;
   }
 
@@ -283,7 +285,7 @@ Tabpage window_get_tabpage(Window window, Error *err)
 /// @return true if the window is valid, false otherwise
 Boolean window_is_valid(Window window)
 {
-  Error stub = {.set = false};
+  Error stub = ERROR_INIT;
   return find_window_by_handle(window, &stub) != NULL;
 }
 
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 760457b54e..a119cd6eac 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -12471,25 +12471,18 @@ static void f_rpcrequest(typval_T *argvars, typval_T *rettv)
     ADD(args, vim_to_object(tv));
   }
 
-  bool errored;
-  Object result;
-  if (!channel_send_call((uint64_t)argvars[0].vval.v_number,
-                         (char *)argvars[1].vval.v_string,
-                         args,
-                         &result,
-                         &errored)) {
-    EMSG2(_(e_invarg2), "Channel doesn't exist");
-    return;
-  }
-
-  if (errored) {
-    vim_report_error(result.data.string);
+  Error err = ERROR_INIT;
+  Object result = channel_send_call((uint64_t)argvars[0].vval.v_number,
+                                    (char *)argvars[1].vval.v_string,
+                                    args,
+                                    &err);
+  if (err.set) {
+    vim_report_error(cstr_as_string(err.msg));
     goto end;
   }
-  
-  Error conversion_error = {.set = false};
-  if (!object_to_vim(result, rettv, &conversion_error)) {
-    EMSG(_("Error converting the call result"));
+
+  if (!object_to_vim(result, rettv, &err)) {
+    EMSG2(_("Error converting the call result: %s"), err.msg);
   }
 
 end:
@@ -19442,7 +19435,7 @@ static void script_host_eval(char *method, typval_T *argvars, typval_T *rettv)
     return;
   }
 
-  Error err = {.set = false};
+  Error err = ERROR_INIT;
   object_to_vim(result, rettv, &err);
   api_free_object(result);
 
diff --git a/src/nvim/os/channel.c b/src/nvim/os/channel.c
index 1670424e4e..5598b485ba 100644
--- a/src/nvim/os/channel.c
+++ b/src/nvim/os/channel.c
@@ -172,40 +172,36 @@ bool channel_send_event(uint64_t id, char *name, Array args)
 /// Sends a method call to a channel
 ///
 /// @param id The channel id
-/// @param name The method name, an arbitrary string
+/// @param method_name The method name, an arbitrary string
 /// @param args Array with method arguments
-/// @param[out] result Pointer to return value received from the channel
 /// @param[out] error True if the return value is an error
-/// @return True if the call was sent successfully, false otherwise.
-bool channel_send_call(uint64_t id,
-                       char *name,
-                       Array args,
-                       Object *result,
-                       bool *errored)
+/// @return Whatever the remote method returned
+Object channel_send_call(uint64_t id,
+                         char *method_name,
+                         Array args,
+                         Error *err)
 {
   Channel *channel = NULL;
 
   if (!(channel = pmap_get(uint64_t)(channels, id)) || !channel->enabled) {
+    api_set_error(err, Exception, _("Invalid channel \"%" PRIu64 "\""), id);
     api_free_array(args);
-    return false;
+    return NIL;
   }
 
   if (kv_size(channel->call_stack) > 20) {
     // 20 stack depth is more than anyone should ever need for RPC calls
-    *errored = true;
-    char buf[256];
-    snprintf(buf,
-        sizeof(buf),
-        "Channel %" PRIu64 " crossed maximum stack depth",
-        channel->id);
-    *result = STRING_OBJ(cstr_to_string(buf));
+    api_set_error(err,
+                  Exception,
+                  _("Channel %" PRIu64 " crossed maximum stack depth"),
+                  channel->id);
     api_free_array(args);
-    return false;
+    return NIL;
   }
 
   uint64_t request_id = channel->next_request_id++;
   // Send the msgpack-rpc request
-  send_request(channel, request_id, name, args);
+  send_request(channel, request_id, method_name, args);
 
   EventSource channel_source = channel->is_job
     ? job_event_source(channel->data.job)
@@ -224,10 +220,12 @@ bool channel_send_call(uint64_t id,
       channel->enabled &&  // the channel is still enabled
       kv_size(channel->call_stack) >= size);  // the call didn't return
 
-  *errored = frame.errored;
-  *result = frame.result;
+  if (frame.errored) {
+    api_set_error(err, Exception, "%s", frame.result.data.string.data);
+    return NIL;
+  }
 
-  return true;
+  return frame.result;
 }
 
 /// Subscribes to event broadcasts
@@ -433,7 +431,9 @@ static bool channel_write(Channel *channel, WBuffer *buffer)
 
 static void send_error(Channel *channel, uint64_t id, char *err)
 {
-  channel_write(channel, serialize_response(id, err, NIL, &out_buffer));
+  Error e = ERROR_INIT;
+  api_set_error(&e, Exception, "%s", err);
+  channel_write(channel, serialize_response(id, &e, NIL, &out_buffer));
 }
 
 static void send_request(Channel *channel,
diff --git a/src/nvim/os/msgpack_rpc.c b/src/nvim/os/msgpack_rpc.c
index d7e3d33c4b..55bc006ad1 100644
--- a/src/nvim/os/msgpack_rpc.c
+++ b/src/nvim/os/msgpack_rpc.c
@@ -30,14 +30,14 @@ WBuffer *msgpack_rpc_call(uint64_t channel_id,
   FUNC_ATTR_NONNULL_ARG(3)
 {
   uint64_t response_id;
-  char *err = msgpack_rpc_validate(&response_id, req);
+  Error error = ERROR_INIT;
+  msgpack_rpc_validate(&response_id, req, &error);
 
-  if (err) {
-    return serialize_response(response_id, err, NIL, sbuffer);
+  if (error.set) {
+    return serialize_response(response_id, &error, NIL, sbuffer);
   }
 
   // dispatch the call
-  Error error = { .set = false };
   Object rv = msgpack_rpc_dispatch(channel_id, req, &error);
   // send the response
   msgpack_packer response;
@@ -47,12 +47,12 @@ WBuffer *msgpack_rpc_call(uint64_t channel_id,
     ELOG("Error dispatching msgpack-rpc call: %s(request: id %" PRIu64 ")",
          error.msg,
          response_id);
-    return serialize_response(response_id, error.msg, NIL, sbuffer);
+    return serialize_response(response_id, &error, NIL, sbuffer);
   }
 
   DLOG("Successfully completed mspgack-rpc call(request id: %" PRIu64 ")",
        response_id);
-  return serialize_response(response_id, NULL, rv, sbuffer);
+  return serialize_response(response_id, &error, rv, sbuffer);
 }
 
 /// Finishes the msgpack-rpc call with an error message.
@@ -112,10 +112,10 @@ WBuffer *serialize_request(uint64_t request_id,
 
 /// Serializes a msgpack-rpc response
 WBuffer *serialize_response(uint64_t response_id,
-                            char *err_msg,
+                            Error *err,
                             Object arg,
                             msgpack_sbuffer *sbuffer)
-  FUNC_ATTR_NONNULL_ARG(4)
+  FUNC_ATTR_NONNULL_ARG(2, 4)
 {
   msgpack_packer pac;
   msgpack_packer_init(&pac, sbuffer, msgpack_sbuffer_write);
@@ -123,11 +123,11 @@ WBuffer *serialize_response(uint64_t response_id,
   msgpack_pack_int(&pac, 1);
   msgpack_pack_uint64(&pac, response_id);
 
-  if (err_msg) {
-    String err = {.size = strlen(err_msg), .data = err_msg};
-    // error message
-    msgpack_pack_bin(&pac, err.size);
-    msgpack_pack_bin_body(&pac, err.data, err.size);
+  if (err->set) {
+    // error represented by a [type, message] array
+    msgpack_pack_array(&pac, 2);
+    msgpack_rpc_from_integer(err->type, &pac);
+    msgpack_rpc_from_string(cstr_as_string(err->msg), &pac);
     // Nil result
     msgpack_pack_nil(&pac);
   } else {
@@ -146,43 +146,43 @@ WBuffer *serialize_response(uint64_t response_id,
   return rv;
 }
 
-static char *msgpack_rpc_validate(uint64_t *response_id, msgpack_object *req)
+static void msgpack_rpc_validate(uint64_t *response_id,
+                                 msgpack_object *req,
+                                 Error *err)
 {
   // response id not known yet
 
   *response_id = 0;
   // Validate the basic structure of the msgpack-rpc payload
   if (req->type != MSGPACK_OBJECT_ARRAY) {
-    return "Request is not an array";
+    api_set_error(err, Validation, _("Request is not an array"));
   }
 
   if (req->via.array.size != 4) {
-    return "Request array size should be 4";
+    api_set_error(err, Validation, _("Request array size should be 4"));
   }
 
   if (req->via.array.ptr[1].type != MSGPACK_OBJECT_POSITIVE_INTEGER) {
-    return "Id must be a positive integer";
+    api_set_error(err, Validation, _("Id must be a positive integer"));
   }
 
   // Set the response id, which is the same as the request
   *response_id = req->via.array.ptr[1].via.u64;
 
   if (req->via.array.ptr[0].type != MSGPACK_OBJECT_POSITIVE_INTEGER) {
-    return "Message type must be an integer";
+    api_set_error(err, Validation, _("Message type must be an integer"));
   }
 
   if (req->via.array.ptr[0].via.u64 != 0) {
-    return "Message type must be 0";
+    api_set_error(err, Validation, _("Message type must be 0"));
   }
 
   if (req->via.array.ptr[2].type != MSGPACK_OBJECT_BIN
     && req->via.array.ptr[2].type != MSGPACK_OBJECT_STR) {
-    return "Method must be a string";
+    api_set_error(err, Validation, _("Method must be a string"));
   }
 
   if (req->via.array.ptr[3].type != MSGPACK_OBJECT_ARRAY) {
-    return "Paremeters must be an array";
+    api_set_error(err, Validation, _("Paremeters must be an array"));
   }
-
-  return NULL;
 }
diff --git a/src/nvim/os/provider.c b/src/nvim/os/provider.c
index 2e7a677793..d4fffaa053 100644
--- a/src/nvim/os/provider.c
+++ b/src/nvim/os/provider.c
@@ -107,12 +107,11 @@ Object provider_call(char *method, Array args)
     return NIL;
   }
 
-  bool error = false;
-  Object result = NIL;
-  channel_send_call(f->channel_id, method, args, &result, &error);
+  Error err = ERROR_INIT;
+  Object result = NIL = channel_send_call(f->channel_id, method, args, &err);
 
-  if (error) {
-    vim_report_error(result.data.string);
+  if (err.set) {
+    vim_report_error(cstr_as_string(err.msg));
     api_free_object(result);
     return NIL;
   }