Show URL that caused invalid feed error

The error may originate from another page in the feed, and only showing
the original URL makes it harder to track down invalid feed issues
(see #967).

src/gpodder/model.py

@@ -214,7 +214,7 @@ class gPodderFetcher(feedcore.Fetcher):
             feed['headers'] = headers
             return feedcore.Result(status, PodcastParserFeed(feed, self, max_episodes))
         except ValueError as e:
-            raise feedcore.InvalidFeed('Could not parse feed: {msg}'.format(msg=e))
+            raise feedcore.InvalidFeed('Could not parse feed: {url}: {msg}'.format(url=url, msg=e))
 
 
 # Our podcast model: