In #867 a new scheme for monitoring access of spinners to callback queues was proposed, ensuring that only a single single-threaded spinner can access a given callback queue.
To maintain backwards compatibility to already released ROS distros, it was decided to keep the old, non-strict error handling. In a new ROS distro (L-turtle) a more strict error checking should be applied, i.e. throwing an exception if starting of the spinner failed.
To this end, commit 91be0e5 needs to be reverted. This will also re-enable some more unittests, which check for throwing of these exceptions.