Fixes v2 return status on unauthorized download

If a user is not allowed to download an image because of a policy,
glance v2 API responds with a  HTTP 200 status and no data instead
of HTTP 403 for no cache only.
The problem is that get_data implementation for notification
proxy is a generator, this situation delays the other proxies
get_data calls (including the policy proxy) for the first time
data is retrieved.
Hence, there is a delay in enforcing policy, so 200 is sent before
the API gets the chance to catch the policy exception.

DocImpact
Closes-Bug: #1326781
Change-Id: I1e50a069a6b7f9eed7160cd5908a5fa30274e227
Merge "Fixes v2 return status on unauthorized download"