[1512] return UPDATE_ERROR within the exception handling block

for now, the effect is the same, and we may want to move the return
back to the common part if we want to handle multiple exceptions
in a unified matter.  But at the moment we don't have the need for it,
and the revised position seemed to be less confusing.

src/lib/python/isc/ddns/session.py

@@ -134,7 +134,7 @@ class UpdateSession:
                              ClientFormatter(self.__client_addr),
                              ZoneFormatter(e.zname, e.zclass), e)
             self.__make_response(e.rcode)
-        return UPDATE_ERROR, None, None
+            return UPDATE_ERROR, None, None
 
     def __get_update_zone(self):
         '''Parse the zone section and find the zone to be updated.