gabble_roster_request_subscription_async: don't ignore async IQ errors
gabble_roster_request_subscription_async: don't ignore async IQ errors

In the process, inline gabble_roster_handle_subscribe.

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