bug#65267: 30.0.50; modifying debug-ignored-errors during startup with --debug-init is broken

[Eli Zaretskii]

> Cc: 65267@debbugs.gnu.org
> Date: Sun, 13 Aug 2023 12:19:31 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Yeah, it's wrong, but I'm not sure how to fix it:
> `--debug-init` needs to change the value of `debug-ignored-errors` while
> `init.el` is executed, but it's not clear how to "revert" to "the value
> it would have if we had not changed it" once we're finished processing
> `init.el`.
> 
> Maybe we should introduce a new `inhibit-debug-ignored-errors` which we
> could let-bind while loading `init.el` without having to modify
> `debug-ignored-errors` itself?

We could document that modifying debug-ignored-errors only works
reliably if done from after-init-hook.  WDYT?