Alter the behavior of opening and closing ports - closing a port range that is not open will not result in an error.
Merge pull request #474 from tasdomas/port_closing_restrictions

Alter the behavior of opening and closing ports

Closing a port range that has not been opened will not result in an error.

This is an outcome of the discussions on the juju-dev mailing list. Furthermore it follows the practices adopted by charm authors, where closing a port that was not open is often expected to be a noop.