Push 'Error' result from event to instance action.

Currently a failed event stores an 'Error' result but in order to see
which actions have failed it would require grabbing each one through the
api to see the events.  This patch adds 'Error' to the message field of
the instance action so that any actions with failures can be seen from
the initial listing of actions on an instance.

Change-Id: Ieeb798c47211ab00fe5b767dc1f18a665549ba01
Remove unused db calls from nova.db.api

* iscsi_target_count_by_host
* iscsi_target_create_safe
* reservation_destroy
* volume_get_iscsi_target_num

Change-Id: I0c357a6398509150e7ac987826341cd1e79f0075
Merge "Remove unused db calls from nova.db.api"
Merge "Push 'Error' result from event to instance action."