Error on permission problem

When the images are not writeable and DEBUG is true, I get this answer when trying to use the plugin:

function () { if (c) { var a = c.length; n(arguments), j ? (l...

The JSON result:

Warning: imagejpeg(): Unable to open '../wp-content/uploads/2012/08/image-210x130.jpg' for writing: Permission denied in /var/www/clients/client4/web48/web/wp-includes/media.php on line 467

[...]

{"success":"<b>&quot;image&quot; (ID 9653): All thumbnails was successfully regenerated in 1,766 seconds.<\/b><br \/> - Thumbnail: 300x200 was deleted.<br \/> - Thumbnail: 650x270 was deleted.<br \/> - Thumbnail: 210x130 was deleted.<br \/> - Thumbnail: 430x270 was deleted."}

It looks weird that the result is a success..

It would be nice if the output could be improved to include something useful ??

https://www.ads-software.com/extend/plugins/force-regenerate-thumbnails/