Administer » System Settings » Debugging and Error Handling
Debugging options are covered in the developer documentation.
Fatal Error Template - Enter the path and filename for a custom Smarty template if you want to define your own screen for displaying fatal errors.
Fatal Error Handler - Enter the path and class for a custom PHP error-handling function if you want to override built-in CiviCRM error handling for your site.
Click Save to save your action or Cancel to cancel it.
If successful you will see the message "Your changes have been saved."