propagate datastore exceptions all the way to northbound

propagate datastore exceptions all the way to northbound

Instead of logging but ultimately ignoring them, as they currently are.

Stumbled across this wile working on (not directly related) NEUTRON-158.

This will also fix the problem raised in NEUTRON-157: We are,
intentionally, changing the OptimisticLockFailedException WARN log to a
DEBUG only.  If it still didn't go through after RETRY_MAX (currently 2)
then it will be rethrown, and that, with this, will go back up to the
driver.

JIRA: NEUTRON-157
Change-Id: I10b7dea0b5698db40ee7cfaa63593fb6b3c573c7
Signed-off-by: Michael Vorburger <vorburger@redhat.com>