* Fix the IQDB lookup to handle error messages returned by IQDB.
* Fix `undefined method `>=' for nil:NilClass` error when trying to upload
ugoiras. Using the preview image for IQDB lookups should be faster and
give the same results as the full image, plus it should work with things
like ugoiras and videos that IQDB can't handle.
Also add an image_url param so that API users can continue using the
full image if needed.