Description:
------------
imagepng()/imagegif()/imagejpeg() function do not report the exact reason why they failed to write to the file.
This makes it difficult to debug, as you know it fails, but it doesn't say why.

Reproduce code:
---------------
<?php

$i=imagecreate(10,10); 
imagepng($i, "/root/test.jpg");

?>

Expected result:
----------------
Warning: imagepng(): Unable to open '/root/test.jpg' for writing : Permission denied in .. on line ..

Actual result:
--------------
Warning: imagepng(): Unable to open '/root/test.jpg' for writing in .. on line ..