If display_errors is set, Admins are not adequately warned.

Within setup.php, we check to see if the PHP setting "display_errors" is enabled and if so, we define "WARN_DISPLAY_ERRORS_ENABLED", which adds a notice to the Notifications page, as well as a warning when running the security report.

The "WARN_DISPLAY_ERRORS_ENABLED" value will only get defined if "display_errors" is enabled in the php.ini, not if $CFG->debugdisplay is set.

Steps to Reproduce:

• In php.ini, set display_errors = 0
• In config.php set $CFG->debugdisplay = 1
• Navigate to Site Administration > Reports > Security Overview and see that "Displaying of PHP Errors" is marked as "OK"