Implements in a Rails engine the Rails 3.2 pattern for dynamically-generated error pages. (see, for example blog.plataformatec.com.br/2012/01/my-five-favorite-hidden-features-in-rails-3-2/)
One motivation for an engine implementation is the fact that the feature requires the Rails production environment configuration. This does not integrate well with an application's test suite.
The other motivation, of course, is it's DRY, and re-usable.
Master branch is now the rails 4 compatible branch, if you need rails 3.2 use the rails3 branch.