When `:check_response => true` in `DB#command` we run the operation and the validate the response using `Support#ok?`. If the resulting operation wasn't successful, `DB#command` will raise a generic `OperationError` containing whatever error message we got back from the MongoDB server. 

For authentication, we don't want that generic error type so we don't ask `DB#command` to validate the response from the server. Instead, we want to make sure the code here raises an `AuthenticationError`. Github's collapsed diff view is hiding the under-belly of this code from you, but I handle this for all auth types in the plumbing that lives around the logic for this specific auth mechanism.

https://github.com/brandonblack/mongo-ruby-driver/blob/32123f7c2d455707378120eadf95f081011274b1/lib/mongo/functional/authentication.rb#L182-L185