Restart the apache web server, via NodeWorx or at the command line:
service httpd restart
Create the error files in the users web space. For example, if you create a 404.html in the user's html directory, 404 errors will show that page.
Each user will be able to create/edit his or her own error pages. If there is no 404.html file in the user's account then a generic 404 error will be displayed.
If you want each site to be able to specify it's OWN error message file names:
To do this you create a .htaccess file in the user's html directory, with one of the following lines, if you only wanted to override 404 errors.