Hi Nick
WSDL is only applicable to SOAP-based web service so you won't have any for REST services.
I haven't personally used SAP's REST adapter but I have used Advantco's. In my experience using that to consume REST services, I just define fault message types in ESR for both sender and receiver service interfaces, and link them via a fault mapping. I've mentioned it in the "Fault handling" section of my blog below. Maybe you can try that approach with SAP's REST adapter.
Consuming RESTful CRUD services with Advantco REST adapter
Another alternative you can try out is to implement some custom error handling as detailed in the blog below.
PI REST Adapter - Custom error handling
Rgds
Eng Swee