Better invalidation handling

[Aaron-P]

If a cache key is invalidated from under an info call for example, we should try to give a better error than 'doesn't exist'.

[Aaron-P]

I'm not sure what other sort of behavior we can have for this, any ideas @IDisposable? A possibility would be to keep track of the expiration time of the entry in javascript and show a different warning if they try to get details for an expired key if we expected it to be expired.