bug#28797: 26.0.90; Improve printing of error on catching file-error in dired

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Thu, 12 Oct 2017 23:16:28 -0400
> Cc: 28797@debbugs.gnu.org, tino.calancha@gmail.com
> 
> > 1. Improving the format of error messages generated by catching error
> > signals from C, so that the errors look almost like readable English
> > instead of a dump of a list.
> > 2. Those errors should generate a backtrace.. right now, a user needs
> > to update the debug-on-message variable to force back traces for such
> > messages which is, first: more cumbersome than doing M-x
> > toggle-debug-on-error and then recreating the error, and second: not
> > a common knowledge.
> 
> Here's a patch, uses error-message-string for #1,

This doesn't handle all of the parts of the complaint, does it?

> condition-case-unless-debug for #2.

This is quite a radical change in very old behavior, so if it is
deemed a good idea, it should go to master.

Thanks.