In particular, we should deal with KeyboardInterrupt (by
just doing 'pass'), as well as ExternalShutdownException
(which can come from the executor).  In all cases, we should
call 'rclpy.try_shutdown()' to cleanup at the end.

Note well that I removed some of the *destroy() calls in
the cleanup.  That's because they weren't correct in all cases,
and to fix them up properly would really require us to have
a nested set of try..except statements.  Given that these are
examples, having that complex set of exception handling didn't
seem like the correct way to go here.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

Signed-off-by: Chris Lalancette <clalancette@gmail.com>