Wrap all asset read operations.

First step in moving assets out of the contentstore (optionally).
https://openedx.atlassian.net/browse/PLAT-77
This test is confusing. I assume it's throwing an error for now b/c we have no means of storing the asset itself through the modulestore mechanism. If so, add that as a comment or better yet, change this test to always test the condition of metadata exists but asset doesn't.