sftp: Quit more cleanly on remote shutdown

Don't issue any operations after g_vfs_backend_force_unmount() is
called.

This prevents messages like the following repeated many times when the
remote sftp-server process quits unexpectedly:
forced_unregister_mount_callback

** (process:3348): WARNING **: Error unregistering mount: Mountpoint not
registered (g-io-error-quark, 16)

https://bugzilla.gnome.org/show_bug.cgi?id=731035