Unified error-handling function

Spun off from #1397956: Automatic usage of all available command plugins

I've been trying to add a lot of error reporting but it could probably be better. Specifically, it can be really hard to figure out whether imports are working or not, and if not why not.

Ideally any errors show up in the watchdog, but some Feeds errors won't. So it might be worth creating a unified error-handling function that can display errors on-screen, in the watchdog, and/or in an email bounce, depending on user preferences. We'd have to be able to capture Feeds errors as well though, which might be tough and require patches there.