> set options -> Build -> No external pages = checked
              -> Build -> No error pages = checked

> Then you can change the links to the warning page to
> the page ou want, with a search & replace within all
> files in a directory, with notepad++ for example

LOL
...or, rather than going through all that, name this catch-all page
"external.html", put it in the project's root folder, and all the built-in
links will point to it!

~BD