Convert UserCreateBranch to use structured errors

UserCreateBranch is returning an error message in the response with a successful status code. This is goes against good practices and makes the error invisible to Praefect. We should update UserCreateBranch to return an error status code with all errors. In the cases where the client needs to handle the error and needs to identify it, we should return a structured error that allows for that.

Fixes: #4227 (closed)