Error output should have lowercase attribute names (#244)
Error output should have lowercase attribute names (#244)

Adding protobuf tags to `errorBody` to get proper lowercased output when marshaled.
gengo/grpc-gateway,ilius/grpc-gateway,ilius/grpc-gateway,tmc/grpc-gateway,Lantame/grpc-gateway,Lantame/grpc-gateway,gengo/grpc-gateway,grpc-ecosystem/grpc-gateway,improbable-io/grpc-gateway,ilius/grpc-gateway,tmc/grpc-gateway,improbable-io/grpc-gateway