SOAP has a standard way of propagating errors, a SOAPFault field in the envelope. This standard also implies a number of things, such as its structure.
RESTful, being an application style, implies much less on how errors should be propagated. There's no concept of an envelope and more and more I believe that the services contract are supposed to be as simple as possible, a very straightforward JSON structure (oh, XML is supposed to be also possible, but I challenge a real use case...).
That being said, simple doesn't mean incomplete or weak. But I can't seem to find a good guidance on how to do this - and worse, it seems to me every API defines it's own way of doing it, resulting in a very heterogenic, savage even, environment.
TL;DR Is there a standard governing RESTful services (even if only proposed) that would give me any guidance with error propagation?