Add some safety to remove_all_listers

The same websocket client object is often used between many different
parts of the software. (for example skills) This will reduce the risk
that all listeners are removed for all events by mistake. It it still
possible toi do this by calling
`WebsocketClient.ws.remove_all_listers()` if there would be need for
this mechanism.

Author: forslund

mycroft/messagebus/client/ws.py

@@ -101,6 +101,14 @@ def remove(self, event_name, func):
         self.emitter.remove_listener(event_name, func)
 
     def remove_all_listeners(self, event_name):
+        '''
+            Remove all listeners connected to event_name.
+
+            Args:
+                event_name: event from which to remove listeners
+        '''
+        if event_name is None:
+            raise ValueError
         self.emitter.remove_all_listeners(event_name)
 
     def run_forever(self):