Fail on error, we are in trouble.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230717145544.194786-6-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: avoid intermediate variable]
Signed-off-by: Eric Blake <eblake@redhat.com>