Throw an exception if there's multiple moderations for the same object.

There's no sensible default behaviour, so you'll need to override
filter_moderated_objects to define what bevahiour you want. What if one
moderation approves an object, and the other rejects it? What if one is
still pending?

Either way, the old code simply picked one moderation at random, and that's
just broken.
Merge branch 'improve_multiple_mods_handling'