A default HTTP error handler that can be used when there's no application available.
Note: this HttpErrorHandler should ONLY be used in DEV or TEST. The way this displays errors to the user is generally not suitable for a production environment.
By default, the implementation of this method delegates to onProdServerError when in prod mode, and onDevServerError in dev mode. It is recommended, if you want Play's debug info on the error page in dev mode, that you override onProdServerError instead of this method.