Handle os.listdir failures in object-updater

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

This patch catches the unhandled exception that can be thrown
by os.listdir if the self.devices directory is inaccessible.

Change-Id: I6293b840916bb63cf9eebbc05068d9a3c871bdc3
Related-bug: 1375348
Merge "Handle os.listdir failures in object-updater"