Default callback for substasks link_exception

        Default callback re-raises the exception inside _run()

def on_error(self, subtask: Greenlet):
        """ Default callback for substasks link_exception

        Default callback re-raises the exception inside _run() """
        log.error(
            'Runnable subtask died!',
            this=self,
            running=bool(self),
            subtask=subtask,
            exc=subtask.exception,
        )
        if not self.greenlet:
            return
        self.greenlet.kill(subtask.exception)