* fix(gateway): log error-notification failures instead of silently swallowing
The last-resort exception handler in _process_message_background() that
sends an error notice to the user caught all exceptions with a bare pass,
leaving zero trace when the notification itself failed. Upgrade to
logger.error(..., exc_info=True) so a failed error-notification send is
debuggable post-mortem.
Salvaged from #6499 by @BongSuCHOI (the logging-upgrade portion only).
* docs: add PR infographic for gateway error-notify logging
