Unfortunately the only way we can know the user_id at this point is
by parsing the error message.
I also refactored the exceptions in openstack_auth to make them use
different classes (but one common superclass).
Partially implements blueprint: allow-users-change-expired-password
Change-Id: Ieceee09db21040b96577db19bd195dc3799e3892
9d98a0c24d