Make REST payloads on errors consistent

[berndbohmeier]

Currently we most of the time return something like {message: "foo", errors: []}, but at least for invalid json something like {messages: []} is returned. We should work out a constent format and always use it.
To change the messages behavior it looks like we have to override https://github.com/marshmallow-code/webargs/blob/dev/src/webargs/flaskparser.py#L114