[WIP] Preview error pages in Symfony 2.6+

#### WIP: We should discuss #4294 first. If it gets merged in 2.3 (and 2.3 is in master), I'd need to rebase this PR. 

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.
Yes, there are this and some more fixes over at #4294, please have a look over there.

Once #4294 is discussed & merged for 2.3, I'll rebase and update this one accordingly.

Thanks!