On the ServerGrove blog there's a new post showing you how to customize those default error pages that show up in a Symfony2 application when an exception is thrown.
The framework produces a very detailed report when in the development environment. However, when you put the application in the production server, you will need to display a user friendly page. Creating a custom page is actually simple. All you need to do is define a controller/action pair that will generate the response. You can either use an existing controller or create one specifically to handle errors.
They include a sample "lean" controller to just handle the output for the exceptions as well as a template to go with it that just has a basic customized message and a status code for the exception. Using it is simple - just add an "exception_listener" setting to your configuration file to point at the new controller.