error: Fix inheritance typos for HTTPError (ProcessingError -> FeedError)

Avoid:

     Traceback (most recent call last):
       ...
       File ".../rss2email/feed.py", line 338, in _check_for_errors
         raise _error.HTTPError(status=status, feed=self)
       File ".../rss2email/error.py", line 166, in __init__
         super(FeedError, self).__init__(feed=feed, message=message)
     TypeError: __init__() got an unexpected keyword argument 'feed'

HTTPErrors occur when we're fetching a feed, so we don't have the
parsed feed instance required by ProcessingErrors.  We should use the
more general FeedError instead, and use the HTTPError class name in
the super() call.

Both of changes come from sloppy copy-paste errors while stubbing out
lots of similar error classes during the huge 066602ef (rss2email:
split massive package into modules, 2012-11-13).

Reported-by: Matěj Cepl <mcepl@redhat.com>
Signed-off-by: W. Trevor King <wking@tremily.us>