dirmgr::storage: Treat a missing blob file as an absent object.

Previously it was counted as a hard error, which would cause an absolute failure to start if a blob file had been deleted improperly -- for example, by a renegade cache-cleaner that had decided to remove the largest files it could find.

Upon encountering a missing blob, we remove it from the database as well: if we did not, then unavailable consensuses could still cause us to try to fetch consensus diffs, because their rows would still be present.

Fixes #1466 (closed).