#133: Fix task cancellation breaking broadcast loop

[not-f-elsner]

• Differentiate between broadcast shutdown and task cancellation
• Minor refactor to make the code more readable
• Improved and simplified logging

Closes: #133

[not-f-elsner]

I looked into simply adding the CancelledError to the exceptions which we except when doing task.result(), but we cannot differentiate between the task itself being cancelled by the handler, and the task being cancelled due to the broker shutting down the loop, hence the slightly more complicated logic for loop shutdown

See the example: https://docs.python.org/3/library/asyncio-task.html#asyncio.Task.cancel