def error(self, code=500, callback=None):
        """ Register an output handler for a HTTP error code. Can
            be used as a decorator or called directly ::

                def error_handler_500(error):
                    return 'error_handler_500'

                app.error(code=500, callback=error_handler_500)

                @app.error(404)
                def error_handler_404(error):
                    return 'error_handler_404'

        """

        def decorator(callback):
            if isinstance(callback, basestring): callback = load(callback)
            self.error_handler[int(code)] = callback
            return callback

        return decorator(callback) if callback else decorator