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. 
Again, like the tech details, but I think some of them can be saved for later. For example, you don't really need to know the name of the normal controller unless you end up using option (2) below (replacing that controller).

```rst
The core TwigBundle has a listener for this event that executes a configurable
controller. This controller has a sensible way of picking one out of the available
set of error templates.
```