Do not crash with "swift list --lh" for Ceph RadosGW.

If 'x-timestamp' header is not present in 'swift list' result,
date string cannot be correctly generated. Check for correct
exception to catch this case and return a special string value.
This is just a workaround for faulty RadosGW behavior.

Change-Id: I172843a2668e33efac00924906ff0dca3e790050
Closes-Bug: 1349528