Ignore exceptions raised in user created tasks

[nadavelkabets]

Addresses #1098.
Currently exceptions raised in user created tasks crash the executor. In my project I had to catch the exception and return it to allow communicating exceptions from the task to the main code.
This pull request mimics the behavior of asyncio, in which exceptions in tasks are ignored and logged upon garbage collection.
The logging part is already implemented, although this code is currently unreachable.
It was only necessary to add a line in the SingleThreadedExecutor to check if the task is related to an entity, since user created tasks are not bounded to any entity.

rclpy/rclpy/rclpy/task.py

Lines 56 to 60 in 4e8b071

	def __del__(self) -> None:
	if self._exception is not None and not self._exception_fetched:
	print(
	'The following exception was never retrieved: ' + str(self._exception),
	file=sys.stderr)

@fujitatomoya

[apockill]

yes thank you for this. I ran into this and didn't know enough about the Task system to know if this was expected behavior, so I ended up wrapping my function in an exception catcher, then re-raising it after getting the future.

[fujitatomoya]

lgtm but i would like to request other maintainers to review, test would be ideal.

CC: @sloretz @ahcorde