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).

Fixes #431.