Explicitly raise OperationalException if markets are not loaded

correctly

freqtrade/pairlist/IPairList.py

@@ -150,6 +150,9 @@ class IPairList(ABC):
         black_listed
         """
         markets = self._exchange.markets
+        if not markets:
+            raise OperationalException(
+                    'Markets not loaded. Make sure that exchange is initialized correctly.')
 
         sanitized_whitelist: List[str] = []
         for pair in pairlist: