If AUTH_USER_MODEL is improperly configured as 'project.customer.User',
the error is:

ValueError: too many values to unpack

Use rather standard Django's error:

ImproperlyConfigured: AUTH_USER_MODEL must be of the form
'app_label.model_name'

These are simpler as they are guaranteed to have an email field which
Oscar uses for authentication.

Django 1.5 makes it optional, but given our Emailbackend being default,
that seems like the sensible way to deal with it.