It's funny how many developers (myself including) return CreateUserResponse
More seriously - service layer need not to know how it would be accessed (REST, SOAP, something else...), so it should not have any influence on the exception processing in the service layer. It is responsibility of the presentation layer to convert service layer exceptions into something understandable to the clients. In case of the REST it may be appropriate HTTP status code (http://www.iana.org/assignments/http-status-codes) and some text message. For example in case of "business" exceptions we include the message from exception object itself and for all others just server error message along with some id that allows us easily find detailed info in logs.