controllers/index: Fix fetchData() return value

Without this the `let promise = controller.fetchData();` will be invalid because the method does not return anything.

Author: Turbo87

app/controllers/index.js

@@ -16,7 +16,7 @@ export default class IndexController extends Controller {
   }
 
   @action fetchData() {
-    this.dataTask.perform().catch(() => {
+    return this.dataTask.perform().catch(() => {
       // we ignore errors here because they are handled in the template already
     });
   }