Right now, our usage of MONGO_UNREACHABLE is somewhat overloaded; we use it to represent both

-> "If this code is reached, there is a logic bug" (i.e. default case in an exhaustive switch statement) 

and 

-> "It wasn't worth implementing this yet and it shouldn't be reachable from production" 

It may be useful to distinguish between the two, especially when reading code. We could add a MONGO_UNIMPLEMENTED to disambiguate.