Ignore errors while fetching subreddit names in subscriptions_filters()

If we can't retrieve subreddit name, just use the user-supplied name. This fixes banned subreddits being impossible to to unfilter or unsubscribe from. A drawback of such approach is that it might be possible to subscribe to a subreddit twice with different casing, however the chance of this is extremely low. Fixes #730, supersedes #745.