script: Unsilence all main thread TaskQueue errors (#34849)

No longer hide errors while queueing tasks on the main thread. This
requires creating two types of `TaskSource`s: one for the main thread
and one that can be sent to other threads. This makes queueing a bit
more efficient on the main thread and more importantly, no longer hides
task queue errors.

Fixes #25688.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/fetch.rs

@@ -196,7 +196,7 @@ pub fn Fetch(
     global.fetch(
         request_init,
         fetch_context,
-        global.task_manager().networking_task_source(),
+        global.task_manager().networking_task_source().to_sendable(),
         None,
     );