Use tp_observe_channels_context_fail if preparing the context fail
Use tp_observe_channels_context_fail if preparing the context fail

This avoid to dispose the context in the None state and so raise a warning.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib