It sounds like adding a check that requires the developer to use the mixin in a certain way doesn't gain us much (we'd be back where we started, only now there are two things that the developer has to remember to do). 

Of the remaining options, I think I'd tend towards just adding some technical documentation (maybe with a copy of the unit test that you use to trigger the exception). I'm not terribly excited about calling the method ahead of time, since it'd add an extra overhead to the 99% of times when you don't go through the 404 handler to catch the one-per-deployment chance that it was configured incorrectly.