Automatic merge from submit-queue

Use response content-type on restclient errors

Also allow a new AcceptContentTypes field to allow the client to ask for
a fallback serialization when getting responses from the server. This
allows a new client to ask for protobuf and JSON, falling back to JSON
when necessary.

The changes to request.go allow error responses from non-JSON servers to
be properly decoded.

@wojtek-t - also alters #28910 slightly (this is better output)