[v4] support error handling flow like gRPC (#158)

* add status package with tests and integrate into response parsing
* improve unit-tests
* improve readme

status/error.go

@@ -0,0 +1,14 @@
+package status
+
+// Error is a thin wrapper around Status that implements the error interface.
+type Error struct {
+	s *Status
+}
+
+func (e *Error) Error() string {
+	return e.s.String()
+}
+
+func (e *Error) HTTPStatus() *Status {
+	return e.s
+}