Return the snapshot or raise a 404 if the snapshot is unknown

def get_snapshot(self, snapshot_id):
        """
        Return the snapshot or raise a 404 if the snapshot is unknown
        """
        try:
            return self._snapshots[snapshot_id]
        except KeyError:
            raise aiohttp.web.HTTPNotFound(text="Snapshot ID {} doesn't exist".format(snapshot_id))