Raise informative exception for non-sortable PK

Raise informative exception for non-sortable PK

An informative exception is re-raised when a primary key value is not
sortable in Python during an ORM flush under Python 3, such as an ``Enum``
that has no ``__lt__()`` method; normally Python 3 raises a ``TypeError``
in this case.   The flush process sorts persistent objects by primary key
in Python so the values must be sortable.

Change-Id: Ia186968982dcd1234b82f2e701fefa2a1668a7e4
Fixes: #4232