This changeset introduces a `WP_Fatal_Error_Handler` class that detects fatal errors and displays a more user-friendly message about the site experiencing technical difficulties.
Websites that have custom requirements in that regard can implement their own fatal error handler by adding a `fatal-error-handler.php` drop-in that returns the handler instance to use, which must be based on a class that inherits `WP_Fatal_Error_Handler`. That handler will then be used in place of the default one. Alternatively, the fatal error handler feature can be completely disable through a constant `WP_DISABLE_FATAL_ERROR_HANDLER`.
Websites that would like to modify specifically the error template displayed in the frontend can add a `php-error.php` drop-in that works similarly to the existing `db-error.php` drop-in. For more granular customization, the fatal error handler also includes new filters `wp_should_handle_php_error`, `wp_php_error_message` and `wp_php_error_args`.
Props afragen, bradleyt, flixos90, ocean90, schlessera, SergeyBiryukov, spacedmonkey, timothyblynjacobs.
See #46130, #44458.
git-svn-id: https://develop.svn.wordpress.org/trunk@44962 602fd350-edb4-49c9-b593-d223f7449a82
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