Reduce scope of the 'error message' and make more-explicit use of the grails render method by specifying the status and text for the body, when applicable.
Update test to account for change in number of 'allowed' methods changed in 295ae0d.
Modify validate to return a status code of 200 (again) when the model is not valid.  Update tests to verify 200 is returned from validate in valid as well as invalid scenarios.  In the test, clarify that the text being returned by validate is not an error code, but rather the error message.