Important: #88824 - Add cache for error page handling

See forge#88824


In order to prevent possible DoS attacks when the page-based error handler is used, the content of the 404 error page is now cached in the TYPO3 page cache. Any dynamic content on the error page (e.g. content created by TypoScript or uncached plugins) will therefore also be cached.

If the 404 error page contains dynamic content, TYPO3 administrators must ensure that no sensitive data (e.g. username of logged in frontend user) will be shown on the error page.

If dynamic content is required on the 404 error page, it is recommended to implement a custom PHP based error handler.