add debug statement

src/cryptocom/exchange/market.py

@@ -1,3 +1,4 @@
+import sys
 from typing import AsyncGenerator, Dict, List
 
 from . import pairs
@@ -98,6 +99,8 @@ class Exchange:
             data = await self.api.get(
                 "public/get-candlestick",
                 {"instrument_name": pair.name, "timeframe": period.value})
+        if not data:
+                print(f'no data for {pair.name}: request was {pair} {period} {endTime}',file=sys.stderr)
         return [Candle.from_api(pair, candle) for candle in data]
 
     async def listen_candles(