Enforce eframe errors are `Send` and `Sync` (#4574)

* Follow-up to https://github.com/emilk/egui/pull/4565

crates/eframe/src/epi.rs

@@ -41,7 +41,7 @@ pub type EventLoopBuilderHook = Box<dyn FnOnce(&mut EventLoopBuilder<UserEvent>)
 #[cfg(any(feature = "glow", feature = "wgpu"))]
 pub type WindowBuilderHook = Box<dyn FnOnce(egui::ViewportBuilder) -> egui::ViewportBuilder>;
 
-type DynError = Box<dyn std::error::Error>;
+type DynError = Box<dyn std::error::Error + Send + Sync>;
 
 /// This is how your app is created.
 ///

crates/eframe/src/lib.rs

@@ -334,7 +334,7 @@ pub fn run_simple_native(
 #[derive(Debug)]
 pub enum Error {
     /// Something went wrong in user code when creating the app.
-    AppCreation(Box<dyn std::error::Error>),
+    AppCreation(Box<dyn std::error::Error + Send + Sync>),
 
     /// An error from [`winit`].
     #[cfg(not(target_arch = "wasm32"))]