#391 : Unhandled messages should now be forwarded as Debug messages when DebugUnhandledMessages setting is enabled

- Created a private subclass UnhandledMessageForwarder
- Forwarder is added to the loggers list when DebugUnhandledMessages setting is enabled
- Forwarder converts UnhandledMessage(s) to Debug messages using an extension method
No need for an extension method. `UnhandledMessageForwarder` is the only one needing to do this, so inline this code inside `UnhandledMessageForwarder` instead.