sloecode crashes when port is already in use

If sloecode is misconfigured to use the an ssh port which is already in use (e.g., by the standard sshd), it will crash with
a back trace. On ubuntu, since this is an unhandled exception, it causes apport to try to report bugs against /usr/bin/twistd.

Instead, sloecode should handle the exception and give an appropriate error message and/or exit code.