main: Flush file in place of closing it, also do error reporting

Apparently on travis OS X systems it crashes when cleaning up streams with
stdout closed:

    (lldb) bt all
    * thread #1: tid = 0x0000, 0x00007fff8703df06 libsystem_kernel.dylib`__pthread_kill + 10, stop reason = signal SIGSTOP
      * frame #0: 0x00007fff8703df06 libsystem_kernel.dylib`__pthread_kill + 10
        frame #1: 0x00007fff93a764ec libsystem_pthread.dylib`pthread_kill + 90
        frame #2: 0x00007fff97c056df libsystem_c.dylib`abort + 129
        frame #3: 0x00007fff97bccdd8 libsystem_c.dylib`__assert_rtn + 321
        frame #4: 0x0000000107a4e106 nvim`uv__close(fd=<unavailable>) + 102 at core.c:521
        frame #5: 0x0000000107a5307d nvim`uv__loop_close(loop=0x00007fff5847c018) + 77 at loop.c:118
        frame #6: 0x0000000107a4d149 nvim`uv_loop_close(loop=0x00007fff5847c018) + 57 at uv-common.c:626
        frame #7: 0x000000010783e5bc nvim`stream_set_blocking(fd=0, blocking=true) + 204 at stream.c:34
        frame #8: 0x000000010795d66b nvim`mch_exit(r=0) + 91 at os_unix.c:147
        frame #9: 0x00000001078d5663 nvim`command_line_scan(parmp=0x00007fff5847c760) + 1779 at main.c:787
        frame #10: 0x00000001078d4393 nvim`main(argc=2, argv=0x00007fff5847c898) + 163 at main.c:249
        frame #11: 0x00007fff8cdd65ad libdyld.dylib`start + 1
        frame #12: 0x00007fff8cdd65ad libdyld.dylib`start + 1

1 files changed, 12 insertions, 3 deletions

diff --git a/src/nvim/os/fileio.c b/src/nvim/os/fileio.c
index 0ed0ae87e3..5d68473982 100644
--- a/src/nvim/os/fileio.c
+++ b/src/nvim/os/fileio.c
@@ -400,9 +400,18 @@ int msgpack_file_write(void *data, const char *buf, size_t len)
   assert(len < PTRDIFF_MAX);
   const ptrdiff_t written_bytes = file_write((FileDescriptor *)data, buf, len);
   if (written_bytes < 0) {
-    emsgf(_("E5420: Failed to write to file: %s"),
-          os_strerror((int)written_bytes));
-    return -1;
+    return msgpack_file_write_error((int)written_bytes);
   }
   return 0;
 }
+
+/// Print error which occurs when failing to write msgpack data
+///
+/// @param[in]  error  Error code of the error to print.
+///
+/// @return -1 (error return for msgpack_packer callbacks).
+int msgpack_file_write_error(const int error)
+{
+  emsgf(_("E5420: Failed to write to file: %s"), os_strerror(error));
+  return -1;
+}