bug#60592: 30.0.50; emacsclient: exit with a different code when the request times out

[Eli Zaretskii]

> From: Sean Whitton <spwhitton@spwhitton.name>
> Date: Thu, 05 Jan 2023 22:47:49 -0700
> 
> When calling 'emacsclient --timeout N --eval' from scripts, it would be
> nice if you could determine for sure when a failure was just due to a
> timeout, rather than, say, a Lisp exception.
> 
> inotifywait(1) exits with code 2 to indicate a timeout.
> emacsclient could do the same.

It would be more portable to have the exception handler send a special
message to emacsclient, so that it would know the reason without
guessing.