Ajax requests shouldn't display errors
|Reported by:||pento||Owned by:|
Similar to #23811, displaying PHP errors in an Ajax response will probably cause a critical error in the client code. Instead, we should hide errors.
This is particularly important as #21663 is unlikely to make WordPress 3.8, which means all folks using WP_DEBUG on PHP 5.5 will be getting warnings they can't do anything about.