Status code for failing validation should be 400, not 500

[goto-bus-stop]

I had tried to make this happen in https://github.com/u-wave/core/pull/416. But it looks like it's not working consistently for all endpoints (if at all). In https://github.com/u-wave/core/issues/421 we got a 500 response with no information on the client, even though the only problem was the input failing validation.