Handle fallible events in `OnionMessenger`

Handle fallible events in `OnionMessenger`

Previously, we would just fire-and-forget in `OnionMessenger`'s event
handling. Since we now introduced the possibility of event handling
failures, we here adapt the event handling logic to retain any
events which we failed to handle to have them replayed upon the next
invocation of `process_pending_events`/`process_pending_events_async`.