When get manifest, the handler will try to retrieve it from storage driver. When storage driver is cloud storage, it can fail due to various reasons even if the manifest exists
(like 500, 503, etc. from storage server). Currently manifest handler blindly return 404 which can be confusing to user.
This change will return 404 if the manifest blob doesn't exist, and return 500 UnknownError for all other errors (consistent with the behavior of other handlers).
Signed-off-by: Yu Wang (UC) <yuwa@microsoft.com>
Once upon a time, we referred to manifests and images interchangably.
That simple past is no more. As we grow, we update our nomenclature and
so follows our code.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
