Support returning errors when creating the app (#4565)

The closure passed to `eframe::run_native` now returns a `Result`,
allowing you to return an error during app creation, which will be
returned to the caller of `run_native`.

This means you need to wrap your `Box::new(MyApp::new(…))` in an
`Ok(…)`.

* Closes https://github.com/emilk/egui/issues/4474

tests/test_viewports/src/main.rs

@@ -22,7 +22,7 @@ fn main() {
 
             ..Default::default()
         },
-        Box::new(|_| Box::<App>::default()),
+        Box::new(|_cc| Ok(Box::<App>::default())),
     );
 }