Handle os.listdir failures in container-updater

While investigating bug 1375348 I discovered the problem
reported there was not limited to the object-auditor.  The
container-updated has similar bugs.

This patch catches the unhandled exception that can be thrown by
os.listdir.

Change-Id: I7eed122bf6b663e6e7894ace136b6f4653db4985
Related-bug: 1375348
Merge "Handle os.listdir failures in container-updater"