Added plan by id route and error handling and organisation

routes/errors.go

@@ -10,3 +10,12 @@ func serverError(w http.ResponseWriter, err error) {
 	log.Printf("received error: {%v}", err)
 	http.Error(w, http.StatusText(code), code)
 }
+
+func methodNotAllowedHandler(w http.ResponseWriter, r *http.Request) {
+	code := http.StatusMethodNotAllowed
+	http.Error(w, http.StatusText(code), code)
+}
+func notFoundHandler(w http.ResponseWriter, r *http.Request) {
+	code := http.StatusNotFound
+	http.Error(w, http.StatusText(code), code)
+}