Making a POST request without `data` element gives unhelpful error

[obmarg]

If you make a POST request without a data element at the top level, you hit this exception:

flump/flump/schemas.py

Line 130 in 1bd9d3a

raise FlumpUnprocessableEntity

Which returns a 422 with errors set to null. This is not very helpful. We should probably return a 422 with actual errors. Maybe FlumpUnprocessableEntity should require error parameters...