Does your extension allow me to set a general rule that when someone is trying to reach a bad url or a page I have deleted, it will go to a custom page, instead of the 404 page?
I'm not sure what you mean by "go to a custom page" in that context. Does "go to" means redirect? or just display a custom 404?
sh404SEF provides a very strong support for 404 handling. It will by default display a very enhanced 404 pages which:
- is displayed in the context of your site (ie in your template and thus with your menus, etc)
- is of course fully customizable
- automatically suggests similar URL: user will see a list of a few URLs that sounds similar to the requested one and can directly click to reach them.
In addition and optionally, it can try to automatically redirect to a similar URL.
See 404 handling documentation on this page and the following for all details.
Lastly, if you are after redirecting, there's also a very strong support with generic rules, much easier to create and modify than .htaccess redirects for instance. See documentation on this page for details.