Return 422 instead of 400 when validation fails

[evillemez]

422 is more descriptive of what is actually happening when validation fails on an object - the entity is unprocessable. The request its self is still ok, it's just the contents that are bad, so 400 is the wrong code for this scenario.

@gliese1337 Will this cause you any issues if I make this change?

[gliese1337]

No problems here. I think it is an excellent idea. Always good to have more fine-grained error information.