Retrieve the Asset for a given sid.

def retrieve_asset(self, sid, default_none=False):
        """
        Retrieve the Asset for a given sid.
        """
        try:
            asset = self._asset_cache[sid]
            if asset is None and not default_none:
                raise SidsNotFound(sids=[sid])
            return asset
        except KeyError:
            raise SidsNotFound(sids=[sid])