You can modify .htaccess file under your domain mapping root to setup custom error. The syntax is like that:
ErrorDocument <3-digit-http-status-code> Action
Following are some samples:
ErrorDocument 500 /cgi-bin/500.html
ErrorDocument 500 "Sorry, our script crashed."
ErrorDocument 500 http://www.yourdomain.com/
ErrorDocument 404 /error/not_found.html
Ref: http://httpd.apache.org/docs/1.3/custom-error.html
In addition, you may setup custom error on your domain via hosting control panel -> site admin -> Set Custom Error