actually, due to this 1-to-1 DB constraint: https://github.com/edx/edx-platform/blob/9be6cf2f256d9f181d7e5b505326a17fcc82dce8/common/djangoapps/external_auth/models.py#L24 it doesn't make sense to handle the MultipleObjectsReturned case.  So I took it out.
Also added logging (level INFO) for the DoesNotExist case (which is the common case).