Comment 4 for bug 1743120

Reviewed: https://review.openstack.org/533752
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=5f881ff030c119b50bf1929b9829f52493ac447c
Submitter: Zuul
Branch: master

commit 5f881ff030c119b50bf1929b9829f52493ac447c
Author: Chris Dent <email address hidden>
Date: Sat Jan 13 16:11:51 2018 +0000

    [placement] use simple FaultWrapper

    Prior to this patch placement was using the nova FaultWrapper
    middleware as a final guard against unexpected exceptions. That
    middleware does more than required and also imports stuff that
    placement does not otherwise require.

    The new middleware is only for exceptions that have fallen through
    previous handling and need to be turned into 500 responses, formatted
    to JSON, and logged.

    While doing this it became clear that the limited Exception catching
    in handler.py was redundant if FaultWrapper and the Microversion
    middlewares (both required for the operation of the system) are
    involved, so that has been removed, with a comment.

    This should not require a microversion as the structure of successful
    responses and expected error responses does not change. What does
    change is that a 500 response will be formatted more as the api-wg
    errors guideline[1] desires.

    [1] http://specs.openstack.org/openstack/api-wg/guidelines/errors.html

    Change-Id: Ib2c3250b964a30239f298cdf13ea2020f205f67d
    Partial-Bug: #1743120