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

examples/hello_world/src/main.rs

@@ -16,7 +16,7 @@ fn main() -> Result<(), eframe::Error> {
             // This gives us image support:
             egui_extras::install_image_loaders(&cc.egui_ctx);
 
-            Box::<MyApp>::default()
+            Ok(Box::<MyApp>::default())
         }),
     )
 }