Return 422 status code in ModelViewSet

For interoperability with EmberData it seems I need to reply with 422 (Unprocessable Entity) instead of 400 (Bad Request) whenever validation errors occur. I have two questions:

• How can I specify the response status code when using a ModelViewSet?
• Why is the 422 not listed in the list of possible return codes?

And bonus:

• Why is EmberData expecting 422? This is not part of the JSONAPi spec, as far as I can see.