fix(messages): incorrect error message splitting and kind #32990

Problem:  Message kind logic for emitting an error message is convoluted
          and still results in emitting an unfinished message earlier than
          wanted.
Solution: Ensure emsg_multiline() always sets the kind wanted by the caller
          and doesn't isn't unset to logic for emitting the source message.
          Caller is responsible for making sure multiple message chunks are
          not emitted as multiple events by setting `msg_ext_skip_flush`...

src/nvim/main.c

@@ -2064,7 +2064,7 @@ static void do_exrc_initialization(void)
       xfree(str);
       if (ERROR_SET(&err)) {
         semsg("Error detected while processing %s:", VIMRC_LUA_FILE);
-        semsg_multiline(err.msg);
+        semsg_multiline("emsg", err.msg);
         api_clear_error(&err);
       }
     }