Fix Notifier async callback error handling - #2096

Summary

Route listener exceptions through Notifier._on_error() when a Notifier is using an asyncio event loop.

With a loop configured, thread-backed receivers currently schedule _on_message_received() using call_soon_threadsafe(). Exceptions raised by synchronous listeners therefore occur later on the event-loop thread and bypass _rx_thread()'s exception handler. Coroutine listeners have the same problem because their tasks are only discarded on completion, so task exceptions are never passed to _on_error().

This change:

• wraps event-loop message dispatch so synchronous listener exceptions populate Notifier.exception and call _on_error()
• applies the same guarded dispatch to file-descriptor based loop readers
• inspects completed coroutine tasks, routes their exceptions through _on_error(), and keeps cancelled tasks quiet
• adds regression tests for both synchronous and asynchronous listener failures while an asyncio loop is configured

The existing no-loop receive-thread behavior is unchanged.

Fixes #1912.