Abstract class which marks an error view.
It's an abstract class instead of an interface,
because you can use it for navigation (which is restricted to classes).
(Since only the final page should be a class and for the rest it's recommended to use interfaces,
it isn't a problem/restriction.)
The class which extends this class will also be used as error-view in case of security violations
(if there is no special error-view configured via
Secured.errorView()
)