CMFActivity.ActivityTool: Set exc_type default value.

This property is only conditionally set, which causes distracting error
when something goes wrong. If we really want to check that the expected
code path was followed there has to be a better check and a more debugging-
friendly error than an AttributeError.

product/CMFActivity/ActivityTool.py

@@ -197,6 +197,7 @@ class Message(BaseMessage):
   active_process_uid = None
   call_traceback = None
   exc_info = None
+  exc_type = None
   is_executed = MESSAGE_NOT_EXECUTED
   traceback = None
   oid = None