Per site error messages

Hi all,

According to the code  
(http://svn.jahia.org/svn/jahia/trunk/core/src/java/org/jahia/bin/JahiaErrorDisplay.java) it should be possible to enable per-site error  
messages.

Does anyone have any idea how this is actually enabled? Should it be  
done though the templates.xml when deploying a new site (if so: how)?

Thanks in advance,

Arjan