Fix some warnings

src/platform_impl/web_sys/event_loop.rs

@@ -265,7 +265,7 @@ fn add_event<T: 'static, E, F>(
         event_ref.cancel_bubble();
 
         handler(&elrs, event);
-    }) as Box<FnMut(E)>);
+    }) as Box<dyn FnMut(E)>);
 
     target.add_event_listener_with_callback(event, &closure.as_ref().unchecked_ref());
     closure.forget(); // TODO: don't leak this.

src/platform_impl/web_sys/window.rs

@@ -62,7 +62,7 @@ impl Window {
         let canvas: HtmlCanvasElement = element.unchecked_into();
         document().body()
             .ok_or_else(|| os_error!(OsError("Failed to find body node".to_owned())))?
-            .append_child(&canvas);
+            .append_child(&canvas).map_err(|_| os_error!(OsError("Failed to append canvas".to_owned())))?;
 
         register(&target.runner, &canvas);