Updating image with invalid operation type raises 500

[Kamil Rykowski]

When trying to update an image using PATCH method I got Internal Server Error when 'op' (operation) has invalid value. We could consider changing this behavior to return some kind of 40x message (400 probably).

Steps to reproduce:
URL: http://localhost:9292/v2/images/<image_id>
Headers:
  X-Auth-Token: XYZ
  Content-Type: application/openstack-images-v2.1-json-patch
Data:
[{"path": "/description", "value": "", "op": "axdd"}]

Result: 500 Internal Server Error

I'm opening it as `Opinion`, as there can be a reason why are we returning 500 instead of some 40x code.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/184023

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/kilo
Review: https://review.openstack.org/190995

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/190995
Committed: https://git.openstack.org/cgit/openstack/glance/commit/?id=abc2a0c1f4be926f742eff57b3d8643d578f8861
Submitter: Jenkins
Branch: stable/kilo

commit abc2a0c1f4be926f742eff57b3d8643d578f8861
Author: Kamil Rykowski <email address hidden>
Date: Mon May 18 13:19:54 2015 +0200

    Change status code from 500 to 400 for image update request

    Invalid 'op' (operation) value provided for image update PATCH request
    results in raising Internal Server Error (500). There is no helpful
    information what went wrong and how to fix this particular request.
    Proposed fix change the status code from 500 to 400 and return proper
    message, so the user won't be confused and will be able to adjust the
    request.

    Change-Id: Ie20a3f350cbaa46c558b4e49b46091d1e4a74807
    Closes-Bug: 1456157
    (cherry picked from commit 9f9629ada550c7ddf66fd03d6604e16a5712881f)