Datafeed history reports failed fetches as errors

GET /api/v1/datafeed/history now answers {"s":"error","errmsg":"history temporarily unavailable"} when the venue fetch fails, for example during a rate limit. It used to answer {"s":"no_data"}, which looked like a market with no candles. The status code stays 200. no_data now only means there are no candles in the range, so retry on error.