Make sure periodic cleanup of instances continues on error

If an instance cleanup fails with an exception, no more instances
will be cleaned up. If the failure is permanent, this will block
other instances from being cleaned up for as long as it remains.

Resolves bug 1192712.

Change-Id: Ibdd69817ee0e6d0963637f38a8be3949e6e053ed
Merge "Make sure periodic cleanup of instances continues on error"