bug#65837: 30.0.50; Debugger in non-main threads

[Eli Zaretskii <eliz@gnu.org>]

> From: Helmut Eller <eller.helmut@gmail.com>
> Date: Sat, 09 Sep 2023 11:15:52 +0200
> 
> When the debugger is entered in a non-main thread, like so
> 
>   emacs -Q --eval '(make-thread (lambda() (debug)))'
> 
> Then debugger-quit doesn't work as usual, instead it prints
> 
>   debugger-quit: No catch for tag: top-level, nil
> 
> or it prints
> 
>   Back to top level
> 
> but doesn't actually close the debugger window.
> 
> Also, debugger-continue doesn't work as usual, instead it sometimes prints:
> 
>   debugger-continue: No catch for tag: exit, nil
> 
> sometimes it prints:
> 
>   Continuing.
> 
> without deleting the debugger window, sometimes it manages to leave the
> debugger.
> 
> If Emacs is started with "emacs -Q -nw" then the behaviour seems to be a
> bit more uniform, but equally useless.

Yes, error handling in non-main threads is generally rudimentary and
barely useful.  For example, if a thread signals an error, it simply
silently exits and leaves the error form in a variable that can be
accessed via thread-last-error.  AFAIR, it is not trivial to improve
the thread error handling significantly, but patches are welcome, of
course.