How to set non Drupal 404 and 403 pages for Drupal?
If you want to catch errors not handled by Drupal, you can do this using .htaccess:

ErrorDocument 403 /error403.html
ErrorDocument 404 /error404.html
ErrorDocument 500 /error500.html


However, if you want to intercept the errors handled by Drupal, and redirect that to an standalone error page, you should be able to do this with Redirect 404.