For Umbraco v9 RC1 - Beta 003
For Umbraco v9 RC2 - Beta 004
For Umbraco v9 RC3 - Beta 005+
Page Not Found Manager is a package that allows content editors to pick an Umbraco Content Page to use as the 404 page for the site. This package also allows for different 404 pages to be used for different sections of the website.
Once installed and running, Page Not Found Manager extends the "Do Something Else" context menu with a new entry for picking a 404 page off of a content node.
This allows for multiple different 404 pages to be configured for different parts of the website.