Wrap all asset read operations.

First step in moving assets out of the contentstore (optionally).
https://openedx.atlassian.net/browse/PLAT-77
Would this comment clear things up?
```
        # Get the asset metadata and have it be found in the modulestore.
        # Currently, no asset metadata should be found in the modulestore. The code is not yet storing it there.
        # If asset metadata *is* found there, an exception is raised. This test ensures the exception is indeed raised.
        # THIS IS TEMPORARY. Soon, asset metadata *will* be stored in the modulestore.
```
There's nowhere to check for the asset's existence if the metadata is in the modulestore, since it's not yet implemented.