Automatically select the App ErrorController.

If the application implements an ErrorController, use that instead of
the core one. This provides an easy convention based way for developers
to customize error handling. If you need plugin controllers, you'll
still have to override the renderer as well.

Refs cakephp/docs#2112
Merge pull request #5401 from cakephp/use-app-error-controller

Automatically select the App ErrorController.