(PUP-2369) Add class to the message we return.

Add the class of the values we're changing from and to in the message
that's being returned. At the very least this will give developers a
hint about the types they're comparing explaining possible weird
behaviour.
Merge pull request #2589 from daenney/add-ensure-type-logging

(PUP-2369) Add class to the message we return.