Fix getting futures exceptions

[decaz]

Access to asyncio.Future.exception() should be protected from unexpected raise of asyncio.CancelledError exception.

[decaz]

@mosquito I see there is another place where exception() method of future is being used:

aiormq/aiormq/base.py

Lines 98 to 101 in 9696466

	def _create_closing_future(self) -> asyncio.Future:
	future = self.__future_store.create_future()
	future.add_done_callback(lambda x: x.exception())
	return future

But I guess it's normal that asyncio.CancelledError can be raised here. Am I right?