refactor(exit): pass error message to preserve_exit() (#22097)

Problem:
1. Some calls to preserve_exit() don't put a message in IObuff, so the
   IObuff printed by preserve_exit() contains unrelated information.
2. If a TUI client runs out of memory or receives a deadly signal, the
   error message is shown on alternate screen and cannot be easily seen
   because the TUI exits alternate screen soon afterwards.

Solution:
Pass error message to preserve_exit() and exit alternate screen before
printing it.

Note that this doesn't fix the problem that server error messages cannot
be easily seen on exit. This is tracked in #21608 and #21843.

src/nvim/ui_client.c

@@ -122,7 +122,9 @@ void ui_client_run(bool remote_ui)
 
 void ui_client_stop(void)
 {
-  tui_stop(tui);
+  if (!tui_is_stopped(tui)) {
+    tui_stop(tui);
+  }
 }
 
 void ui_client_set_size(int width, int height)