feed: Rename 'processing error' -> 'feedparser warning'

A few folks have stumbled over this one, thinking that the error
message meant the feed wasn't parsed [1,2,3].  All we're actually
doing is passing along a feedparser warning, so adjust our prefix
accordingly and drop the level from ERROR to WARNING.  Hopefully this
makes it clearer that:

* We still process these feeds, despite the boxo exception.  If
  feedparser can parse the feed despite the exception, it will do so.
* We probably can't fix these issues in rss2email.  You need to fix
  them in the feed itself, or adjust feedparser to deal with the
  busted feed.

[1]: http://article.gmane.org/gmane.mail.rss2email/168
[2]: https://github.com/wking/rss2email/issues/37
[3]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=760963

Reported-by: Arun Persaud <apersaud@lbl.gov>
Reported-by: Joey Hess <joeyh@debian.org>
Signed-off-by: W. Trevor King <wking@tremily.us>