Show more error detail for invalid application creation requests

[seh]

When marathonctl app create fails for some reason other than the application already existing (status code 409) or the proposed ID being invalid (status code 422), marathonctl doesn't display any of the detail provided by the server to clarify what went wrong, per the comments in file app.go here.

At minimum, marathonctl should show the content of the response object's "message" field, and perhaps the other errors associated with each path in the "details" collection.