No user message reported when migration of multiattach volume fails

[Rajat Dhasmana]

When we migrate a multiattach volume that is associated with more than 1 instance, we fail in the background since it's an async request without notifying the user about the failure.
A user message would be helpful to determine the reason for failure, in this case, a multiattach volume being attached to more than one instance.

Also we send a request to nova and fail whereas we can determine this case in cinder itself and fail fast without sending a request to nova.

c-vol logs

Jun 17 16:10:53 Ubuntu cinder-volume[539025]: ERROR cinder.volume.manager novaclient.exceptions.BadRequest: Swapping multi-attach volumes with more than one read/write attachment is not supported. (HTTP 400) (Request-ID: req-cd7d97af-b5b9-4067-a159-5232913a8123)
Jun 17 16:10:53 Ubuntu cinder-volume[539025]: ERROR cinder.volume.manager

n-api logs

Jun 17 16:10:53 Ubuntu <email address hidden>[539265]: DEBUG nova.api.openstack.wsgi [req-c2303d08-4090-42ab-b75a-95c24adb9426 req-cd7d97af-b5b9-4067-a159-5232913a8123 service nova] Returning 400 to user: Swapping multi-attach volumes with more than one read/write attachment is not supported. {{(pid=539265) __call__ /opt/stack/nova/nova/api/openstack/wsgi.py:943}}