Tiago,
This php.ini entry can be found in /etc/e-smith/templates/etc/php.ini/30ErrorHandling .
You just might edit this file and run a ´/sbin/e-smith/expand-template /etc/php.ini´ and restart your webserver. Or you do it the neat way and create /etc/e-smith/templates-custom/etc/php.ini/ , copy the file 30ErrorHandling to this directory, edit the file and perform the same last steps (expand-template and restart webserver).
That's it!