client: Fix up error handling in find_enclosing_mount

2a0dc68c7eeb ("client: Remove inconsistencies between async and sync
variants") introduced a regression which causes find_enclosing_mount to
segfault if a non-NULL GError** is passed in.  Fix this.  Also, only
call g_dbus_error_strip_remote_error () on GErrors that have come from
dbus.

https://bugzilla.gnome.org/show_bug.cgi?id=739075