404 errors are not handled #702

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Describe the bug

While I was checking #515 , I tried to check the latest state of 404 pages and found out that 404 errors are not handled anymore. While https://processing.org/404 is working fine, a page / URL that doesn't exist doesn't make use of the 404 page: https://processing.org/missing-page

Expected behavior

404 errors should render https://processing.org/404 page with the requested URL.

Screenshots

Browser

Firefox, Other (please specify)

Browser version

Operating system

Smartphone details

Additional context

No response