Error output should have lowercase attribute names

[nathanborror]

The current error output is titlecase ({"Error": "...", "Code": "..."}) when it should be lowercase ({"error": "...", "code": "..."}). This appears to be due to errorBody implementing proto.Message interface but lacking protobuf tags needed to marshal correctly. When I patch errorBody with the following I end up with correct behavior:

type errorBody struct {
    Error string `protobuf:"bytes,1,name=error" json:"error"`
    Code  int    `protobuf:"bytes,2,name=code" json:"code"`
}

[yugui]

LGTM. Thank you for your contribution.

I'll fix the test failure with tip version of Go later.