Properly handle shutdown (canceled coroutines)

This will imrove shutdown behavior

freqtrade/exchange/exchange_ws.py

@@ -102,8 +102,12 @@ class ExchangeWS:
     def _continuous_stopped(
             self, task: asyncio.Task, pair: str, timeframe: str, candle_type: CandleType):
         self._background_tasks.discard(task)
-        result = task.result()
-        logger.info(f"{pair}, {timeframe} Task finished {result}")
+        if task.cancelled():
+            result = "cancelled"
+        else:
+            result = task.result()
+
+        logger.info(f"{pair}, {timeframe} Task finished: {result}")
         self._klines_scheduled.discard((pair, timeframe, candle_type))
 
     async def _continuously_async_watch_ohlcv(