Update the cookbook entry for custom error pages

This IMO clarifies the available approaches, removes minor inaccuracies and also explains how to use the new preview feature from symfony/symfony#12096.

Changes are partially based on [a blog post](http://www.webfactory.de/blog/symfony2-exception-handling-and-custom-error-pages-explained) I have written a while ago.
your versionadded text is far too long. It should be a simple short sentence. And there should be no empty line before it