bug#35188: catch gnus-summary-save-article errors, even in nndoc groups

[積丹尼 Dan Jacobson]

 o [gnus-summary-save-article]

has a bug. If the article is gone, the user sees a flash, but is then
prompted to save it anyway.

Opening nndoc server on nndoc:nnml:mail.misc-26341-ephemeral...done
No such file or buffer: #<killed buffer>
No such article (may have expired or been canceled)
Appended to /tmp/ee

Apparently this only happens when reading ^D digests.

I'm not saying you need to fix why the article is gone.
I'm just saying you need to detect that the error occurred and abort
prompting the user to fill in a file name.

Gnus v5.13
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian