Store purge should ignore errors

Purging of expired entries from stores is a pretty involved process, especially with RocksDBStore. When there's a problem unmarshalling the expired bucket or deleting an expired key, the purge task bails out immediately, without processing the remaining keys. To make matters worse, the exception it not logged anywhere. The only sign that something is wrong is a growing store (in the case of RocksDBStore, a growing number of SST files).