fault messages for consumed REST service

I am defining a REST service 'consumed' from a 3rd party.  The 3rd party system has defined the structure for error messages but I do not see any means of defining that fault message structure on a consumer REST service.  I need the actual application error message being returned by the vendor, not just an HTTP status code.   In PeopleBooks (Integration Broker > Using Peoplecode to Manage REST Service Operations > Using Peoplecode to Manage Consumer REST Service Operations) there is sample peoplecode of sending a message and then checking the response but it seems to indicate that the structure