Raise sensible error when styles are misconfigured

I just spent a considerable amount of time trying to figure out why retina_rails wasn't working for me. Turns out I hadn't specified the image's format in my style definitions. retina_rails just quietly accepted that but then derived completely nonsensical filenames, such that everything broke.

This change will instead raise a clear error message describing how to fix the problem, right on include time. This would have saved me considerable work. :)