RUBY-529 raise exception for multiple attempts to auth on the same db

Server side, MongoDB will log out the first user when a second authentication 
is issued for the same database. In the past we've handled this by just
quietly updating the authentication credentials we have stored for that db.

The driver spec for auth now requires that we raise an explicit exception so 
that users don't accidentally log out previous authenications applied to the 
database unknowingly.