Author overlay part deux

We're in the process of removing the concept of author group from the application.

https://www.pivotaltracker.com/story/show/79384948
I have on a handful of occasions only, and mostly as I'm messing with things in development, not in production. I remember running into this when trying to deploy a demo instance in a short amount of time, or a separate instance for training purposes, for example.

Still, instead of doing nothing, I'd like the down migration to at least throw, so I don't get into a situation where a down migration succeeds, but then blocks me from migrating forward again because the `paper_id` column on the `authors` table already exists.