XF 1.4 How to create "Now Leaving ..." and "Error 404" pages

I was wondering how I could create a custom page for whenever someone clicks a link on my site, there's a "Now leaving" page at which the user could click continue or go back. I was also looking for a way to make a custom "Error 404" page using XenForo pages. I looked for addons that do this but couldn't find anything.

My website is www.venturemc.net