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/keyboard_events/src/main.rs

@@ -10,7 +10,7 @@ fn main() -> Result<(), eframe::Error> {
     eframe::run_native(
         "Keyboard events",
         options,
-        Box::new(|_cc| Box::<Content>::default()),
+        Box::new(|_cc| Ok(Box::<Content>::default())),
     )
 }