feat: error when action handler is missing

libraries/relay.ts

@@ -24,6 +24,13 @@ export class Relay<TRoutes extends Route[]> {
     }
   }
 
+  /**
+   * Override relay url configuration.
+   */
+  set url(value: string) {
+    this.config.url = value;
+  }
+
   /**
    * Retrieve a route for the given method/path combination which can be further extended
    * for serving incoming third party requests.