bug#42865: 28.0.50; Add new 'copy-region-quietly' defcustom

[Juri Linkov]

>> This looks like the existing function 'user-error' that
>> doesn't enter the debugger when 'debug-on-error' is t.
>> Does the name 'user-message' make more sense?
>
> Yeah, that makes sense.

Then like 'signal' uses 'ERROR-SYMBOL' as an error symbol, not string,
'user-message' should use a symbol as well, e.g. in 'indicate-copied-region':

  (user-message
   'indicate-copied-region
   "Saved text until \"%s\"" (buffer-substring-no-properties (- mark len) mark))

then users put a property on that symbol:

  (put 'indicate-copied-region 'inhibit-message t)

to disable the message.