In that case you need some kind of a double caching. I don't think that this is current possible.
From the cache perspective you need to have a soft expiration like "if expired try to refresh from datastore, if fail use the expired value again for X times"
That sound to me that you need a business logic in front, i.e. a REST service which do this job.
But I think this is a cool feature and you might open a enhancement request for that