cron: fix warn message

cron.pl

@@ -32,8 +32,10 @@ for my $item (@files) {
 }
 
 for my $printer (@files) {
-    system("$REPO/install.pl --update $printer")
-        or warn "Error while updating $printer";
+    system("$REPO/install.pl --update $printer");
+    if ($? != 0) {
+        warn "Error while updating $printer";
+    }
 }