Raises an exception if the given app setting is not defined.

def require_setting(self, name: str, feature: str = "this feature") -> None:
        """Raises an exception if the given app setting is not defined."""
        if not self.application.settings.get(name):
            raise Exception(
                "You must define the '%s' setting in your "
                "application to use %s" % (name, feature)
            )