zdtm: don't ignore hook's errors

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -844,7 +844,7 @@ EOF
 
 	if [ -x "${test}.hook" ]; then
 		echo "Executing cleanup hook"
-		"${test}.hook" --clean
+		"${test}.hook" --clean || return 2
 	fi
 
 	if [ -n "$AUTO_DEDUP" ]; then