This is interesting. I guess if restoring the column without the data puts the DB in an invalid state, I think we should fail more aggressively. I'd rather have `db-migrate` throw when I try to migrate down than have it succeed and then `app.js` blow up or whatever. 

Maybe we can signal it with `exports.down = undefined` and `db-migrate` could actually use that to implement a `--safe` flag that would warn before un-downable migrations or something?