I dont think its a good idea to force security with error messages. If you are in the process of sending and the wallet forces you to create a backup for the change address you might just dump a backup somewhere in order to continue what you are trying to do. The code has enforced the rule, but did not solve anything. The user might not even remember that they still have to create a proper backup and just forget about it. The wallet would remind me anyway, so whats the big deal.