Introduce error handling in the FemtoVG and Skia renderers (#2402)

Avoid unwrap() and expect() and instead propagate errors all the way
down to run_event_loop(), show(), and hide() in the Slint AIP.

api/node/native/lib.rs

@@ -498,7 +498,7 @@ declare_types! {
             let this = cx.this();
             let window = cx.borrow(&this, |x| x.0.as_ref().cloned());
             let window_adapter = window.ok_or(()).or_else(|()| cx.throw_error("Invalid type"))?;
-            window_adapter.hide();
+            window_adapter.hide().unwrap();
             Ok(JsUndefined::new().as_value(&mut cx))
         }