EmptyModlist exception was generated by pwpolicy2-mod when modifying
policy priority only. It was because the priority attribute is stored
outside of the policy entry (in a CoS entry) and there was nothing
left to be changed in the policy entry.
This patch uses the new exception callbacks in baseldap.py classes
to catch the EmptyModlist exception and checks if there was really
nothing to be modified before reraising the exception.
