How do I customize the "Page Not Found" message?

When a visitor goes to my site with a link to a page that doesn't exist, they get a "Page Not Found" page with a link to login or create the page. How do I customize that message?

Customizing this message can be done, but it does require some minor layout customization. The "Page Not Found" is a page object provided to your site's Layout for display just like any other page - except that its created on the fly and doesn't really exist. You can test for it using the page.exists property. In your Layout, you might do something like this to show an existing page named PageNotFound on your site instead of EditMe's standard message:

<div id="pageContent">
  <!-- show the PageDoesNotExist page if the page doesn't exist  -->
  <ss editme="if !page.exists ;; cdata  site.getPage('PageDoesNotExist').content"/>
  <!-- show page content for an existing page -->
  <ss editme="if page.exists">
     <!-- your layout will already contain something like these two lines -->
    <ss editme="if page.isScript ;; process page"/>
    <ss editme="if !page.isScript ;; cdata page.content"/>
  </ss>
</div>