screen.lua: dump payload on handler failure

For debugging failures like:

    test/functional/helpers.lua:240: test/functional/ui/screen.lua:898:
    bad argument #1 to 'unpack' (table expected, got number)

    test/functional/helpers.lua:240: test/functional/ui/screen.lua:708:
    attempt to index local 'item' (a number value)

ref #10804

2 files changed, 8 insertions, 3 deletions

diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index 73f3c4c917..c718a37021 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -304,7 +304,7 @@ local function nvim_feed(input)
     local written = module.request('nvim_input', input)
     if written == nil then
       module.assert_alive()
-      error('nvim_input returned nil (Nvim process terminated?)')
+      error('crash? (nvim_input returned nil)')
     end
     input = input:sub(written + 1)
   end
@@ -592,7 +592,7 @@ end
 
 -- Checks that the Nvim session did not terminate.
 function module.assert_alive()
-  eq(2, module.eval('1+1'))
+  assert(2 == module.eval('1+1'), 'crash? request failed')
 end
 
 local function do_rmdir(path)
diff --git a/test/functional/ui/screen.lua b/test/functional/ui/screen.lua
index 02be714e0d..0e3b903398 100644
--- a/test/functional/ui/screen.lua
+++ b/test/functional/ui/screen.lua
@@ -615,7 +615,12 @@ function Screen:_redraw(updates)
       local handler_name = '_handle_'..method
       local handler = self[handler_name]
       if handler ~= nil then
-        handler(self, unpack(update[i]))
+        local status, res = pcall(handler, self, unpack(update[i]))
+        if not status then
+          error(handler_name..' failed'
+            ..'\n  payload: '..inspect(update)
+            ..'\n  error:   '..tostring(res))
+        end
       else
         assert(self._on_event,
           "Add Screen:"..handler_name.." or call Screen:set_on_event_handler")