Character and Learner both have a verify_from() method that raises different exceptions with the same name

We have:
* `Character.verify_from()` that raises `cryptography.exceptions.InvalidSignature`
* `Learner.verify_from()` that raises `Learner.InvalidSignature`
In the context of an object that is both a Character and a Learner, it seems extremely confusing.

_Originally posted by @cygnusv in https://github.com/nucypher/nucypher/pull/2177#discussion_r474227403_