Quarantine objects with busted metadata.

Before, if you encountered an object with corrupt or missing xattrs,
the object server would return a 500 on GET, and wouldn't quarantine
anything. Now the object server returns a 404 for that GET and the
corrupted file is quarantined, thus giving replication a chance to fix
it.

Change-Id: Ib1d7ab965391742c88fde3d83dc0b5afe85bada9