eframe web: Better panic handling (#2942)

* Refactor: remove extra store of events

* Remove unnecessary extra function

* Refactor: simplify event registering

* Store panic summary

* egui_demo_app: move web-part to own module

* index.html: await

* Properly unsubscribe from events on panic

* Better error handling

* Demo app html: hide the wasm canvas and show an error message on panic

* egui_demo_app: add panic button to test panic response on web

* fix typo

* Use a constructor to create WebHandle

* Refactor: less use of locks in the interfaces

* More consistent naming

crates/egui_demo_app/src/web.rs

@@ -0,0 +1,57 @@
+use eframe::{
+    wasm_bindgen::{self, prelude::*},
+    web::AppRunnerRef,
+};
+
+use crate::WrapApp;
+
+#[wasm_bindgen]
+pub struct WebHandle {
+    runner: AppRunnerRef,
+}
+
+#[wasm_bindgen]
+impl WebHandle {
+    /// This is the entry-point for all the web-assembly.
+    ///
+    /// This is called once from the HTML.
+    /// It loads the app, installs some callbacks, then returns.
+    #[wasm_bindgen(constructor)]
+    pub async fn new(canvas_id: &str) -> Result<WebHandle, wasm_bindgen::JsValue> {
+        // Redirect tracing to console.log and friends:
+        eframe::web::WebLogger::init(log::LevelFilter::Debug).ok();
+
+        // Make sure panics are logged using `console.error`.
+        console_error_panic_hook::set_once();
+
+        let web_options = eframe::WebOptions::default();
+        let runner = eframe::start_web(
+            canvas_id,
+            web_options,
+            Box::new(|cc| Box::new(WrapApp::new(cc))),
+        )
+        .await?;
+
+        Ok(WebHandle { runner })
+    }
+
+    #[wasm_bindgen]
+    pub fn destroy(&self) {
+        self.runner.destroy();
+    }
+
+    #[wasm_bindgen]
+    pub fn has_panicked(&self) -> bool {
+        self.runner.panic_summary().is_some()
+    }
+
+    #[wasm_bindgen]
+    pub fn panic_message(&self) -> Option<String> {
+        self.runner.panic_summary().map(|s| s.message())
+    }
+
+    #[wasm_bindgen]
+    pub fn panic_callstack(&self) -> Option<String> {
+        self.runner.panic_summary().map(|s| s.callstack())
+    }
+}