Better error handling of 'fetch_feeds'

Today I ran into the issue that, a feed url has a problem which '/feed/' redirects to '/feed', and then '/feed' redirects to '/feed/'. That caused 'fetch_feeds' cron never ends, and when I checked that, plenty of them were running.

I think we also need consider what we do for redirects. As far as I can tell, some scrapers don't handle redirects, JFYI.