Improve the error messages in the Executor class.
Improve the error messages in the Executor class.

In particular, make sure to use 'throw_from_rcl_error'
as much as possible.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@openrobotics.org>

ros2/rclcpp,ros2/rclcpp