feat(ui): no delay for errors with ext_messages (#33693)

Problem:  Delay for reading a message may be unwanted for ext_messages,
          and can be done by the implementation. Empty completion source
          error message is not distinguishable as such.
Solution: Only delay without ext_messages enabled. Emit empty completion
          source message as an error.

src/nvim/fileio.c

@@ -3041,7 +3041,7 @@ int buf_check_timestamp(buf_T *buf)
         }
         msg_clr_eos();
         msg_end();
-        if (emsg_silent == 0 && !in_assert_fails) {
+        if (emsg_silent == 0 && !in_assert_fails && !ui_has(kUIMessages)) {
           ui_flush();
           // give the user some time to think about it
           os_delay(1004, true);