When nnrss can't fetch a feed an error is signalled.  This patch makes
nnrss print a warning instead.

When you use 'g' from the Group buffer, the current behavior prevents the
rest of your feeds from being checked.  The root cause is the use of
`(error)' in mm-url-insert-file-contents-external.  This patch just adds a
condition-case in nnrss.el.

I don't know whether I used the right -message function.  What's the
difference between nnheader-message and gnus-message?