deal with more errors from feed updates and feed-image

Here's some cleanups and fixes that have been piling up in my tree.
The major features here are in the three most-recent commits, for showing notices when feeds have had trouble updating, fixing migrations of mysql tables, and the slightly more robust feed-image/favicon handling.
The feed-images handling still isn't ideal, but for the time-being it should at least restore/preserve some icons, rather than let them all go blank when fvicon.com is down.