Add ObjectActionFailed exception and make Instance use it
Add ObjectActionFailed exception and make Instance use it

The obj_load_attr() code in Instance slipped through review with
just a "raise Exception()" for the case where the load cannot
complete. This adds a new exception for the case, fixes the
Instance.obj_load_attr() method, and adds a test to verify it.

Related to blueprint compute-api-objects

Change-Id: I73c966deb3c498019370eb83ade64bff2c48aec0

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects