bug#65897: 29.1.50; Debugging errors surfaced through emacsclient is hard

[Eli Zaretskii]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Tue, 12 Sep 2023 15:42:37 -0400
> 
> Toy example:
> 1. emacs -Q
> 2. (server-start)
> 3. (add-hook 'before-make-frame-hook (lambda () (error "frame hook error")))
> 4. emacsclient -c
> 
> The output of emacsclient is:
> 
> Waiting for Emacs...
> *ERROR*: frame hook error
> 
> That's all, there is no way to get more information about the error that
> happened.
> 
> Ideally, when emacsclient encounters an error and debug-on-error is
> non-nil, it should cause Emacs to enter the debugger.

I'm not sure we can allow entering the debugger from server-execute.
That function is documented as being called from timers and process
filters, so entering the debugger might be unsafe.

Stefan, any comments?