Ensure activity and decision tasks cannot progress on a closed workflow

This is a second barrier because I'm a little nervous about this and I
don't want moto/swf to make any activity progress while in the real
world service, it's strictly impossible once the execution is closed.
Python doesn't seem to have any nice way of freezing an object so here
we go with a manual boundary...

moto/swf/exceptions.py

@@ -119,3 +119,8 @@ class SWFDecisionValidationException(SWFClientError):
             prefix.format(count) + "; ".join(messages),
             "com.amazon.coral.validate#ValidationException"
         )
+
+
+class SWFWorkflowExecutionClosedError(Exception):
+    def __str__(self):
+        return repr("Cannot change this object because the WorkflowExecution is closed")