Capture exception when delete a volume detached

Currently, the cinder api validate volume state before validating volume
attach status, so the VolumeAttached exception will never be raised. this
patch change the order of validation to fix this issue.

Change-Id: I41451f5f9d8e60b3cfc1a8004b704285fc3ddc2b
Merge "Capture exception when delete a volume detached"