with-demoted-errors (was: [Emacs-diffs] trunk r114172: * lisp/saveplace.el (load-save-place-alist-from-file): Demote errors.)

[Karl Fogel <kfogel@red-bean.com>]

From: Stefan Monnier <monnier@iro.umontreal.ca>
>> +                      ;; This is with-demoted-errors, but we want to
>> +                      ;; mention save-place in any error message.
>> +                      (condition-case err
>> +                        (car (read-from-string
>> +                              (buffer-substring (point-min) (point-max))))
>> +                        (error (message "Error reading save-place-file: %S" err)
>> +                               nil)))
> 
>I remember having similar desires in other places.  Should we add
>a "format" argument to with-demoted-errors?

I'm not sure I understand the original comment in saveplace.el,
actually.  Does "mention save-place" mean something special there?

The error string already says "save-place-file", and it will print `err'
too...  In other words, why couldn't one just use `with-demoted-errors'
in that spot right now?  Another way to say it is: the BODY argument to
`with-demoted-errors' can format anything in its error messages that it
want to, so I don't see why that code in saveplace.el isn't using
`with-demoted-errors' already.

Not a facetious question; I'm sure I'm just missing something obvious,
but having pondered I can't think of what it is, so am asking.

-Karl