Treat expired/not-yet-valid directory objects as Errors.

Doing this will make us treat caches that send us these objects as not-working, and close circuits to them instead of trying over and over.

The case where we add a document from the cache requires special handling: it isn't actually a error to find an expired document in our cache (unless the passage of time itself is erroneous, which is a debatable proposition at best).

Closes #431 (closed).