Check return of _load_state
Check return of _load_state

In error scenarios there might be no loadable state available.
This patch adds some robustness handling this scenarios
by checking the result and possible bailing out.

Change-Id: I02e2731b14bec22c22db08d60d8d15db1911a84e
Signed-off-by: Andreas Florath <b15c342f1de186996bcca5e1a23cd684d2fadb32@florath.net>

openstack/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder