Open Stack

Hi All!
Currently I am working on this bug https://bugs.launchpad.net/nova/+bug/1168011 which says we have to change  error message displayed  when attaching a volume fails. Currently it catches all operation errors that libvirt can raise and assume that all of them are the source of device being busy. You can find the source of this code here https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L1160. I have few questions on this bug
 
1.	what kind of error message and other required info should we include in the exception to make it look more 	generalized instead of the current one ?
 
2.	Should we raise separate exception for "Device is Busy" or a single general exception would work fine ?
 
3.	If we need separate exception for device being busy what would be the equivalent libvirt error code  for that
 
I would like to have your feedback/suggestions to proceed further with this bug.
 
Regards,
Siva