You can find more detail on this procedure in the Domino manual (PDF). If you don't have access to the server, you'll have to ask someone who does.

1. From the Notes workstation, choose File - Database - New.
2. Enter the name of the web server.
3. Select 'Domino Web Server Configuration Template'.
4. Enter a title for the database and name it DOMCFG.NSF. Click OK.
5. Add an entry called Anonymous to the database and give it 'Reader' access.
6. Determine which database will store your custom error forms.
7. Create a form that contains the information you want to display. If you're working with a virtual server, select 'Available to Public Access Users' on the Security tab. Save the form.
8. Repeat the last step for each kind of error message.
9. In the Domino Configuration database, choose Create - Mapping Error Message. If you're working with a virtual server, choose Virtual Server Settings and enter the IP address.
10. For each type of error enter the target database and form name in the Target Database File Name and Target Database Form fields. Save your changes.
11. Assign 'Author' access to the server that stores the database.