telepathy: Make group changes truly asynchronous and report errors properly

Take advantage of GroupDetails.change_group() being async, and wait for the
underlying Telepathy operation to complete before returning from it. This
allows us to propagate errors properly, rather than just printing
them as warnings on the terminal.

This also includes changes to not notify of changes to Tpf.Persona.groups
until Telepathy has notified us of the change. This should prevent groups
changing in the UI if the underlying operation has actually failed.

Helps: https://bugzilla.gnome.org/show_bug.cgi?id=671662