Respond with 412 after upload if image was deleted.

When updating the image after upload, we should return an
HTTPPreconditionFailed instead of a 500 error if the image cannot
be found.

This will also provide more information to nova when this occurs
during snapshotting.

Fixes bug 1176174

Change-Id: I270ee129470323dae3919ed0a8c335eae7c24061
Merge "Respond with 412 after upload if image was deleted."