empty handler check when unregister cancel errors + corrections (#577)

Author: ccnlui

alpaca_trade_api/stream.py

@@ -422,7 +422,8 @@ def register_handler(self, msg_type, handler, *symbols):
 
     def unregister_handler(self, msg_type, *symbols):
         for symbol in symbols:
-            del self._handlers[msg_type][symbol]
+            if symbol in self._handlers[msg_type]:
+                del self._handlers[msg_type][symbol]
 
 
 class CryptoDataStream(_DataStream):