tf2_ros_py: Ignore ExternalShutdownException in background thread (backport #930)

[mergify]

Description

I've seen this exception when Ctrl-C-ing a program with TransformListener that uses background thread:

[exploration_evaluator-1] [occupancy_mapper-19] Exception in thread Thread-1 (run_func):
[exploration_evaluator-1] [occupancy_mapper-19] Traceback (most recent call last):
[exploration_evaluator-1] [occupancy_mapper-19]   File "/usr/lib/python3.12/threading.py", line 1073, in _bootstrap_inner
[exploration_evaluator-1] [occupancy_mapper-19]     self.run()
[exploration_evaluator-1] [occupancy_mapper-19]   File "/usr/lib/python3.12/threading.py", line 1010, in run
[exploration_evaluator-1] [occupancy_mapper-19]     self._target(*self._args, **self._kwargs)
[exploration_evaluator-1] [occupancy_mapper-19]   File "/opt/ros/kilted/lib/python3.12/site-packages/tf2_ros/transform_listener.py", line 115, in run_func
[exploration_evaluator-1] [occupancy_mapper-19]     self.executor.spin()
[exploration_evaluator-1] [occupancy_mapper-19]   File "/opt/ros/kilted/lib/python3.12/site-packages/rclpy/executors.py", line 374, in spin
[exploration_evaluator-1] [occupancy_mapper-19]     self.spin_once()
[exploration_evaluator-1] [occupancy_mapper-19]   File "/opt/ros/kilted/lib/python3.12/site-packages/rclpy/executors.py", line 968, in spin_once
[exploration_evaluator-1] [occupancy_mapper-19]     self._spin_once_impl(timeout_sec)
[exploration_evaluator-1] [occupancy_mapper-19]   File "/opt/ros/kilted/lib/python3.12/site-packages/rclpy/executors.py", line 951, in _spin_once_impl
[exploration_evaluator-1] [occupancy_mapper-19]     handler, entity, node = self.wait_for_ready_callbacks(
[exploration_evaluator-1] [occupancy_mapper-19]                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[exploration_evaluator-1] [occupancy_mapper-19]   File "/opt/ros/kilted/lib/python3.12/site-packages/rclpy/executors.py", line 921, in wait_for_ready_callbacks
[exploration_evaluator-1] [occupancy_mapper-19]     return next(self._cb_iter)
[exploration_evaluator-1] [occupancy_mapper-19]            ^^^^^^^^^^^^^^^^^^^
[exploration_evaluator-1] [occupancy_mapper-19]   File "/opt/ros/kilted/lib/python3.12/site-packages/rclpy/executors.py", line 827, in _wait_for_ready_callbacks
[exploration_evaluator-1] [occupancy_mapper-19]     raise ExternalShutdownException()
[exploration_evaluator-1] [occupancy_mapper-19] rclpy.executors.ExternalShutdownException

This PR silences this exception.

Is this user-facing behavior change?

No

Did you use Generative AI?

No

---

This is an automatic backport of pull request #930 done by Mergify.

[ahcorde]

Pulls: #931
Gist: https://gist.githubusercontent.com/ahcorde/a36c9e64f7c0087e24b781b8725a4f35/raw/a3a1ffbca6b9b5a9ca18e7026919114bf159a69c/ros2.repos
BUILD args: --packages-above-and-dependencies tf2_ros_py
TEST args: --packages-above tf2_ros_py
ROS Distro: lyrical
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/19156

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