[CSI] Fix CSI Backup to correctly cleanup on failures
[CSI] Fix CSI Backup to correctly cleanup on failures

Signed-off-by: Grant Griffiths <7561d7b6fa252e14d948ef730f22c68c9d861f6b@portworx.com>

libopenstorage/stork,libopenstorage/stork,libopenstorage/stork