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/wgpu_integration.rs

@@ -182,7 +182,7 @@ impl WgpuWinitApp {
         storage: Option<Box<dyn Storage>>,
         window: Window,
         builder: ViewportBuilder,
-    ) -> Result<&mut WgpuWinitRunning, egui_wgpu::WgpuError> {
+    ) -> crate::Result<&mut WgpuWinitRunning> {
         crate::profile_function!();
 
         #[allow(unsafe_code, unused_mut, unused_unsafe)]
@@ -272,7 +272,7 @@ impl WgpuWinitApp {
         };
         let app = {
             crate::profile_scope!("user_app_creator");
-            app_creator(&cc)
+            app_creator(&cc).map_err(crate::Error::AppCreation)?
         };
 
         let mut viewport_from_window = HashMap::default();