There are a couple of ways you can either change the wording of an error message that someone sees when visiting your site or send them to a certain web page if they run into an error.

1. Install the Apache Error Message application

For accounts created before Dec 06 installation is located under the
ValueApps/Web Application section

For accounts created after Dec 06 it is located under 
Applications/Install & Manage/Web Application section

To install, click on the Apache Error Message link. Agree to the
terms, select which error you are working with, and then specify
either a message to display or a URL to redirect to when a visitor
encounters the error.

To add more error messages, just install the application again 
foreach error and provide the message or URL as before.

OR

If your're familiar with working from the Linux command line,
you can SSH into your account, create an .htaccess file if
it doesn't exist, or edit the existing one to add the directive:

ErrorDocument 404 http://yourdomain.com/errorpage.html

OR

ErrorDocument 404 "Sorry, the page you have requested is not available on this site."

For different errors, just add another line and replace the 404
with whatever error you want to modify and add the desired message or URL.