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. 
It's "override showAction, just the findTemplate method or createResponse". The findTemplate is explained more in detail ("which locates which template should be rendered"), so I think I need to separate that with a comma?