[Ideas] Error handler and/or theme page assignment for page not found

Right now, if a page should be under an eSpace, but isn't, the user gets an ugly white + Times New Roman screen that says "page not found", equal to the uncaught exception page. We need a way to override this, either with a theme, or through a "page not found" exception that we can add in exception flows. That will allow us to customize the screen, as well as use it as a single point of reference for redirects when we move pages, instead of sprinkling entries or dummy pages every time we make a move.

J.Ja