Here it comes...wait for it...wait for it...unit tests would help here!
If the function which previously returned something meaningful but now returns None were being unit tested, the test would immediately fail. Then you'd only be out about 30 seconds of your life (time between test failing and realizing your mistake). Its a good start at the least.
But if we're going to take the syntactic route, perhaps requiring a return statement would be better. Again, hard to do this late in the game. Yep, once you ship a language you're screwed :)