Improve cookbook entry for error pages in 2.3+

This PR cherry-picks the improvements from #4293 that apply to 2.3 as well (almost all!).

It also addresses some of the considerations in #4181. 
We already use it in a few places (for example [here](http://symfony.com/doc/current/book/internals.html)). I would like to use it in headlines when we would apply them. But at least we should be as consistent as possible.