process_uri should throw CarrierWave::DownloadError

Seriously? Another pull for URLs? Yes. (And there's at least one more coming after this...)

Given that we're [already using `CarrierWave::DownloadError`s](https://github.com/jnicklas/carrierwave/blob/master/lib/carrierwave/mount.rb#L356) for [validation under ActiveRecord](https://github.com/jnicklas/carrierwave/blob/master/lib/carrierwave/validations/active_model.rb#L33) (who knew?), process_uri should probably be throwing one of those, so that people are able to show invalid URL errors with the same already-available code that shows 404 errors and protocol errors.