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

crates/eframe/src/native/glow_integration.rs

@@ -313,7 +313,7 @@ impl GlowWinitApp {
                 raw_window_handle: window.window_handle().map(|h| h.as_raw()),
             };
             crate::profile_scope!("app_creator");
-            app_creator(&cc)
+            app_creator(&cc).map_err(crate::Error::AppCreation)?
         };
 
         let glutin = Rc::new(RefCell::new(glutin));