In at least one case (non-unique po-number), we return a text/plain response. In the interest of consistency, we should return application/json conforming to the error schema from the raml project, just as RMB does for 422s.
We should also make sure we're specifying a useful value in the 'code' field, e.g. poNumberNotUnique. These codes will likely be mapped to error messages presented to the user in the UI.
- All (or as many as possible) error conditions return an application/json response conforming to the error schema from the raml project
- useful values are provided in the 'code' field