Change Jingle error namespace to current draft.
Change Jingle error namespace to current draft.

While this will mean that we no longer send errors in the old namespace
even when using the draft v0.15 dialect, this doesn't matter in
practice: Gabble doesn't actually do anything differently based on the
errors it receives.

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