GabblePrivateTubesFactory: fix misleading error message
GabblePrivateTubesFactory: fix misleading error message

Cut and paste error from GabbleIMFactory.

community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,community-ssu/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,community-ssu/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble