errors: add errors coder (#946)

* add errors coder

* rename internal http to httputil

* add errors proto

examples/helloworld/client/main.go

@@ -36,6 +36,7 @@ func callHTTP() {
 	}
 	log.Printf("[http] SayHello %s\n", reply.Message)
 
+	// returns error
 	reply, err = client.SayHello(context.Background(), &pb.HelloRequest{Name: "error"})
 	if err != nil {
 		log.Printf("[http] SayHello error: %v\n", err)