Use gnus-error to report mail-source failures

Use gnus-error to report mail-source failures

* lisp/gnus/mail-source.el (mail-source-fetch): Instead of querying
the user on mail sources errors -- in effect asking "do you want to
continue, or halt the process?" -- log the error with `gnus-error',
severity 5. The query didn't provide any meaningful control; error
reporting is all that's needed.
(mail-source-ignore-errors): Obsolete this option; users can see the
error or not by configuring `gnus-verbose'.
* doc/misc/gnus.texi (Mail Source Customization): Remove mention of
the above option from the manual.