rclpy: Exception never retrieved when using MultiThreadedExecutor and Timers

Bug report

Required Info:

• Operating System: Ubuntu 20.04.4
• Installation type: binaries
• Version or commit hash: foxy
• DDS implementation: default
• Client library (if applicable): rclpy

Steps to reproduce issue

I am encountering an issue with exceptions being thrown in timer callbacks using MultiThreadedExecutor going silent.

The sample shows that when a an exception is generated in a timer callback using the SingleThreadExecutor, implicitly I believe, the exception rises if uncaught, as expected.

When using the MultiThreadedExecutor the exception gets lost and silently fails, sometimes causing head scratching.

#!/usr/bin/env python3
import rclpy
from rclpy.executors import MultiThreadedExecutor
from rclpy.node import Node


class TestNode(Node):
    def __init__(self):
        super().__init__('zf9p')
        self.tmr = self.create_timer(1, self.tmr_callback)

    def tmr_callback(self):
        1/0
        print('Timer callback fired')


def single_executor():
    test_node = TestNode()

    while rclpy.ok() is True:
        rclpy.spin(test_node)

    test_node.destroy_timer(test_node.tmr)


def multi_executor():
    test_node = TestNode()

    # Moved to multi threaded executor so none of Actions block publishing of msgs
    executor = MultiThreadedExecutor(num_threads=1)
    executor.add_node(test_node)

    while rclpy.ok() is True:
        executor.spin()

    test_node.destroy_timer(test_node.tmr)


if __name__ == '__main__':
    rclpy.init()

    # single_executor()  # un comment to verify proper operation 
    multi_executor()

    rclpy.shutdown()

Expected behavior

Unhandled exception to rise to the top

$ python3 timer_example
Traceback (most recent call last):
  ...
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 343, in _execute_timer
    await await_or_execute(tmr.callback)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 118, in await_or_execute
    return callback(*args)
  File "timer_example", line 13, in tmr_callback
    1/0
ZeroDivisionError: division by zero

Actual behavior

Exception seems to silently fail not handled, but after 10 or 30 seconds the following message does show.

$ python3 timer_example
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero
The following exception was never retrieved: division by zero

Additional information

This looks to be similar to https://github.com/ros2/rclpy/issues/296, but not for certain.