Ok, you wanna use .htaccess
I use this for error-handling:
# "400 Bad request"
ErrorDocument 400 /400.html
# "401 Authorization required"
ErrorDocument 401 /401.html
# "403 Forbidden"
ErrorDocument 403 /403.html
# "404 Not found"
ErrorDocument 404 /404.html
# "500 Internal server error"
ErrorDocument 500 /500.html
Should work for you too... adjust to fit your needs