Fixing regression around not authorized error messages.

Hey @durran, it seems that this regressed in 1.5.0-stable. This code exists in 1.4.0-stable and master but not in 1.5.0-stable -- I just updated my staging environment with 1.5.0 yesterday and this happened again (see https://github.com/mongoid/moped/pull/174 where I first added it).