Provide more details on Bad Requests

[ahal]

When something is wrong with the request payload, autograph just returns a generic 400 - Bad Request error that doesn't give any details over what is wrong with the request.

In one case, the problem was that files had invalid characters (~) in them. In another, it's that something wasn't base64 encoded. It's often impossible to guess without needing to ask someone to dig through the logs for an error. A simple description of the problem in the response would go a long way!