Print proper message when user not exist in authn request

[orenbm]

We currently do not verify that the user exists in authentication (and login) requests
with the default authenticator. This means that in case a user sends an authn request
with default authenticator and a Conjur user that does not exist we will write an
InvalidCredentials message to the log. This doesn't tell the story properly and a
RoleNotFound message will be better.

Note: this message will not be returned in the response and will only be written
to the log.