DRb::start_service fails to detect used port
When I call DRb.start_service('druby://localhost:1234') and port 1234 is already in use, I silently end up with a broken DRb service. start_service should detect that the port is already in use and raise an exception.
I've attached a small test case. Starting it once works fine. But keeping it running and starting it again from another shell should fail. It does not.