create a volume with invalid snapshot should report 404 instead of 500

[jichenjc]

in v2.1 API, when create a volume through os-volumes
if the snapshot ID is invalid, we should report 404 , but now
exception.SnapshotNotFound is not correctly handled

[jichenjc (jichenjc)]

this was raised by Feodor Tersin at
https://review.openstack.org/#/c/165244/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/173965
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=1e163aca281ec27e70ef987c504a24052a9f8807
Submitter: Jenkins
Branch: master

commit 1e163aca281ec27e70ef987c504a24052a9f8807
Author: jichenjc <email address hidden>
Date: Fri Apr 10 19:49:47 2015 +0800

    Catch SnapshotNotFound exception at os-volumes

    When create a volume ,nova allow user input a snapshot id,
    it will get the snapshot according to the ID. We
    didn't handle exception.SnapshotNotFound thrown from
    cinder layer.

    Change-Id: Icb1708d12dabb82ccba89edcf0f5edbd1a1c82ee
    Closes-Bug: 1444542