Raise error if channel cannot be subscribed (spacewalk-channel)

When the ``spacewalk-channel`` command is used with the ``--add`` switch, this change adds a test if a channel can actually be added to the current system by checking if the parent channel of the one that should be subscribed is currently subscribed to that system. An error is raised if not and the command will exit with an exit code of 1 instead of signaling success by returning 0.