Unquiesce an Instance after a Failed Volume Snapshot

During "snapshot_volume_backed()" an instance is quiesced ( fsfreeze ) before all attached volumes are iterated and a snapshot request sent to Cinder. If Cinder cannot complete a snapshot it will raise an exception. The function "snapshot_volume_backed()" does not catch this exception and the instance is never thawed. This leaves the instance in a frozen, read-only, inconsistent state until the filesystem can be thawed.