fix(runtime): display proper error when throwing in event handlers (#32663)

## Summary

- Fixes `"Uncaught null"` being displayed instead of the actual error
when throwing in `unload`, `beforeunload`, `load`, or process
`exit`/`beforeExit` event handlers
- Root cause: `reportException` in JS calls `op_dispatch_exception`
which stores the real exception in `ExceptionState` and terminates
execution. The Rust-side dispatch functions then checked
`tc_scope.exception()` which returned `null` (the termination exception)
instead of the stored one
- Fix: use `exception_to_err` which properly checks `ExceptionState` for
a dispatched exception before falling back to `tc_scope.exception()`

Before:
```
error: Uncaught null
```

After:
```
error: Uncaught Error: foo
globalThis.addEventListener("unload", () => { throw new Error("foo"); });
                                                    ^
    at file:///...$deno$eval.mts:1:53
    at innerInvokeEventListeners (ext:deno_web/02_event.js:781:9)
    ...
```

Closes #29087

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: bartlomieju

libs/core/error.rs

@@ -1313,7 +1313,7 @@ pub(crate) fn exception_to_err_result<'s, 'i, T>(
   Err(exception_to_err(scope, exception, in_promise, clear_error))
 }
 
-pub(crate) fn exception_to_err<'s, 'i>(
+pub fn exception_to_err<'s, 'i>(
   scope: &mut v8::PinScope<'s, 'i>,
   exception: v8::Local<'s, v8::Value>,
   mut in_promise: bool,

libs/core/lib.rs

@@ -90,6 +90,7 @@ pub use crate::convert::FromV8;
 pub use crate::convert::FromV8Scopeless;
 pub use crate::convert::ToV8;
 pub use crate::cppgc::GarbageCollected;
+pub use crate::error::exception_to_err;
 pub use crate::extensions::AccessorType;
 pub use crate::extensions::Extension;
 pub use crate::extensions::ExtensionArguments;

runtime/worker.rs

@@ -946,8 +946,9 @@ impl MainWorker {
     let undefined = v8::undefined(tc_scope);
     dispatch_load_event_fn.call(tc_scope, undefined.into(), &[]);
     if let Some(exception) = tc_scope.exception() {
-      let error = JsError::from_v8_exception(tc_scope, exception);
-      return Err(error);
+      return Err(deno_core::exception_to_err(
+        tc_scope, exception, false, true,
+      ));
     }
     Ok(())
   }
@@ -963,8 +964,9 @@ impl MainWorker {
     let undefined = v8::undefined(tc_scope);
     dispatch_unload_event_fn.call(tc_scope, undefined.into(), &[]);
     if let Some(exception) = tc_scope.exception() {
-      let error = JsError::from_v8_exception(tc_scope, exception);
-      return Err(error);
+      return Err(deno_core::exception_to_err(
+        tc_scope, exception, false, true,
+      ));
     }
     Ok(())
   }
@@ -978,8 +980,9 @@ impl MainWorker {
     let undefined = v8::undefined(tc_scope);
     dispatch_process_exit_event_fn.call(tc_scope, undefined.into(), &[]);
     if let Some(exception) = tc_scope.exception() {
-      let error = JsError::from_v8_exception(tc_scope, exception);
-      return Err(error);
+      return Err(deno_core::exception_to_err(
+        tc_scope, exception, false, true,
+      ));
     }
     Ok(())
   }
@@ -1032,8 +1035,9 @@ impl MainWorker {
     let ret_val =
       dispatch_beforeunload_event_fn.call(tc_scope, undefined.into(), &[]);
     if let Some(exception) = tc_scope.exception() {
-      let error = JsError::from_v8_exception(tc_scope, exception);
-      return Err(error);
+      return Err(deno_core::exception_to_err(
+        tc_scope, exception, false, true,
+      ));
     }
     let ret_val = ret_val.unwrap();
     Ok(ret_val.is_false())
@@ -1056,8 +1060,9 @@ impl MainWorker {
       &[],
     );
     if let Some(exception) = tc_scope.exception() {
-      let error = JsError::from_v8_exception(tc_scope, exception);
-      return Err(error);
+      return Err(deno_core::exception_to_err(
+        tc_scope, exception, false, true,
+      ));
     }
     let ret_val = ret_val.unwrap();
     Ok(ret_val.is_true())

tests/specs/run/event_handler_throw_error/__test__.jsonc

@@ -0,0 +1,19 @@
+{
+  "tests": {
+    "throw_in_unload": {
+      "args": "run throw_in_unload.ts",
+      "output": "error: Uncaught Error: unload error\n[WILDCARD]",
+      "exitCode": 1
+    },
+    "throw_in_beforeunload": {
+      "args": "run throw_in_beforeunload.ts",
+      "output": "error: Uncaught Error: beforeunload error\n[WILDCARD]",
+      "exitCode": 1
+    },
+    "throw_in_load": {
+      "args": "run throw_in_load.ts",
+      "output": "error: Uncaught Error: load error\n[WILDCARD]",
+      "exitCode": 1
+    }
+  }
+}

tests/specs/run/event_handler_throw_error/throw_in_beforeunload.ts

@@ -0,0 +1,3 @@
+globalThis.addEventListener("beforeunload", () => {
+  throw new Error("beforeunload error");
+});

tests/specs/run/event_handler_throw_error/throw_in_load.ts

@@ -0,0 +1,3 @@
+globalThis.addEventListener("load", () => {
+  throw new Error("load error");
+});

tests/specs/run/event_handler_throw_error/throw_in_unload.ts

@@ -0,0 +1,3 @@
+globalThis.addEventListener("unload", () => {
+  throw new Error("unload error");
+});