Validator should check if it is being called on a user entity

Problem/Motivation

A programmer could apply the AlternativeUserEmailsValidator to a non-user entity. This will have unexpected results because the validator does not check to see if it is operating on a user entity. (It used to, but this check was mistakenly removed. The check is necessary because the validator uses the entity id as the user ID to exclude the current user from the query, so if the entity is not a user entity, the wrong ID will be used.)

I propose not only restoring the check but renaming the variables to make it clear this is a $user, not a generic $entity, and throwing exceptions if the fundamental criteria for running the validator are not met. Silently failing is dangerous if a programmer expects validation and no error is returned, and these exceptions can only be triggered by programmers applying the validator to new code.

Another approach would be to inject the current user and then get the UID that way.