Fix 'message-not-sent' handler. add missing class

src/common/connection_handlers_events.py

@@ -1153,6 +1153,10 @@ class MessageSentEvent(nec.NetworkIncomingEvent):
     name = 'message-sent'
     base_network_events = []
 
+class MessageNotSentEvent(nec.NetworkIncomingEvent):
+    name = 'message-not-sent'
+    base_network_events = []
+
 class MessageErrorEvent(nec.NetworkIncomingEvent):
     name = 'message-error'
     base_network_events = []