Render useful errors when album artists are invalid

[robbevp]

Fixes #1000

Draft PR with a failing test - I'll pick this up later again

• I've added tests relevant to my changes.

[robbevp]

@chvp I added a POC for how we could fix this situation: if we use accepts_nested_attributes_for, we (1) don't have update raise when assigning an invalid association and (2) get errors that are more specific about the association (though you still don't know which of the album artists is invalid).

Two other options I see:

• We recreate this behaviour ourselves in the controller
• We allow album_artists_attributes rather than album_artists and require the client to send the data in the expected format (note that this would need to include {id: 1, _destroy: true } for records that should be removed

I think I have a preference for the last option, our web client already handles a bunch of logic when editing data so it feels more at home there than in the controllers. This also means that requests could be more granular and actually update one album_artist at a time, without needing to resend all the other data