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. 
Not sure - there's nothing special in the TwigBundle config, so I'd say it's the "default" priority and your own one must be better than that (i. e. non-default)?