TDD & Application Use Cases
Creating "success" test cases
Create two test cases by running a request and recording the blank response, then altering it to a desired actual response for a specific request.
Creating "fail" test cases
An error message returned from a service is a contract between the server and the calling client. Its string is significant for the client or caller to respond. So, go ahead and define an error code, and use that in an error use case.