Fix error message in cinder/api/v2/volumes.py
Fix error message in cinder/api/v2/volumes.py

Minor fix.

Change-Id: I1269e271581cc523368b3e59fe4b8ced4febf9de
Closes-Bug: #1479598

scottdangelo/RemoveVolumeMangerLocks,cloudbase/cinder,Hybrid-Cloud/cinder,leilihh/cinder,scality/cinder,openstack/cinder,cloudbase/cinder,scottdangelo/RemoveVolumeMangerLocks,winndows/cinder,NetApp/cinder,Nexenta/cinder,apporc/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,apporc/cinder,nikesh-mahalka/cinder,dims/cinder,NetApp/cinder,takeshineshiro/cinder,bswartz/cinder,phenoxim/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,tlakshman26/cinder-new-branch,leilihh/cinder,takeshineshiro/cinder,eharney/cinder,JioCloud/cinder,duhzecca/cinder,j-griffith/cinder,Nexenta/cinder,eharney/cinder,mahak/cinder,Datera/cinder,tlakshman26/cinder-new-branch,petrutlucian94/cinder,nexusriot/cinder,mahak/cinder,Paul-Ezell/cinder-1,CloudServer/cinder,potsmaster/cinder,nexusriot/cinder,CloudServer/cinder,duhzecca/cinder,JioCloud/cinder,scality/cinder,Hybrid-Cloud/cinder,openstack/cinder,tlakshman26/cinder-https-changes,manojhirway/ExistingImagesOnNFS,bswartz/cinder,potsmaster/cinder,phenoxim/cinder,tlakshman26/cinder-https-changes,sasukeh/cinder,j-griffith/cinder,winndows/cinder,nikesh-mahalka/cinder,sasukeh/cinder,manojhirway/ExistingImagesOnNFS,Paul-Ezell/cinder-1,dims/cinder,ge0rgi/cinder,petrutlucian94/cinder,Datera/cinder