return power state ERROR instead of an exception

iboot's _power_status() could raise an exception if it doesn't get the
expected response from the iboot command. A new IBootOperationError is raised
if the response isn't a list, and we return ERROR power state if the returned
list doesn't have the power status information.

Change-Id: I4b88c24a77a4f46464cf36224878b4e3205f646c
Partial-Bug: #1267693
Merge "return power state ERROR instead of an exception"