Properly handle corner cases of _prepare_async() called twice
Properly handle corner cases of _prepare_async() called twice

- If it's called twice before the first call returned, emit an error.
- If it's called twice on an already-prepared object, just successfully
  return.

GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets