Implement entitlement/product status caching.

Use cached status when server is unreachable.

Fix incorrect product not in server response logging.
Logic error causing incorrect logging that an unentitled product was not
present in the response from the server.
Re-throw network error if no status cache exists.

This is really unlikely but just in case, we will let the network
exception bubble up for now.

Fixed a variable name and removed an unecessary method.
Merge pull request #544 from candlepin/dgoodwin/status-cache

Dgoodwin/status cache