Catching None data error in nasdaq_trader.

Author: simongarisch

pandas_datareader/nasdaq_trader.py

@@ -106,4 +106,6 @@ def get_nasdaq_symbols(retry_count=3, timeout=30, pause=None):
                     retry_count -= 1
                     time.sleep(pause)
 
+    if _ticker_cache is None:
+        raise RemoteDataError # no data collected
     return _ticker_cache