422 HTTP errors thrown when a wrong entity is passed into REST

As of #2827084: EntityNormalizer::denormalize should not throw UnexpectedValueException, but \Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException, so we get a 422 response instead of 400 we distinguish between bad requests and unprocessable entities by throwing a 422 error for POST/PATCH requests when the passed in entity has valid encoded content (like valid JSON), but invalid data, like missing bundle keys or other required keys (depending on fields etc..). Previously we threw 400 responses for all errors, which is now just thrown when the actual request is invalid, for example invalid JSON is sent.