scripts: return non zero if save_mountpoint() fails

In such case, we must not proceed: without saved mountpoint, we risk
losing data when restarting automount service (think about tmpfs).
Currently, zero will be returned as exit status of preceeding echo.
https://jira.sw.ru/browse/PSBM-44207



Signed-off-by: Eugene Kolomeetz <klm@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>