Comment 7 for bug 1571666

Reviewed: https://review.openstack.org/332098
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=1d8eabaff1cff9efbc122b68a54b94745e001cd6
Submitter: Jenkins
Branch: master

commit 1d8eabaff1cff9efbc122b68a54b94745e001cd6
Author: Pavel Bondar <email address hidden>
Date: Tue Jun 21 14:34:57 2016 +0300

    Do not rewrite original exception for IPAM part 2

    If subnet allocation fails on _save_subnet stage, then rollback mechnism
    is executed. Currently if exception is raised on rollback stage it
    replaces original exception. But original exception is needed by
    retry wrappers to work properly also it is more informative for user.

    Using _safe_rollback method to prevent replacing original exception with
    one from rollback stage.

    Closes-Bug: #1571666
    Change-Id: I388272820451bb117281948fef0000bb46a951d4