Customize your site’s error pages

The default error messages Drupal gives your users when they navigate to a web page that they don’t have permission to view or a page that is not found can be intimidating and not really useful. Thus you have the possibility to configure specific Error pages. To do so you have to create pages with a useful message for the cases that the access to a certain element is denied and the cases where a specific page can't be found. After you created the error pages you can link them under Configuration > System > Site Information in the section "Error pages".