Fix error message in imagebackend
Fix error message in imagebackend

When use RBD as storage backend and try to resize instance
to smaller flavor, resize fails with wrong message.
This fix it.

Closes-Bug: #1544599
Change-Id: I62add102bbf3c7064583892dbcc91d4683afe96e

Juniper/nova,openstack/nova,cyx1231st/nova,alaski/nova,hanlind/nova,hanlind/nova,rahulunair/nova,rahulunair/nova,Juniper/nova,BeyondTheClouds/nova,cernops/nova,jianghuaw/nova,phenoxim/nova,mikalstill/nova,rajalokan/nova,mahak/nova,CEG-FYP-OpenStack/scheduler,hanlind/nova,mikalstill/nova,vmturbo/nova,Juniper/nova,CEG-FYP-OpenStack/scheduler,klmitch/nova,mahak/nova,cyx1231st/nova,sebrandon1/nova,klmitch/nova,vmturbo/nova,cloudbase/nova,zhimin711/nova,rahulunair/nova,phenoxim/nova,gooddata/openstack-nova,cernops/nova,sebrandon1/nova,jianghuaw/nova,cernops/nova,rajalokan/nova,bigswitch/nova,gooddata/openstack-nova,gooddata/openstack-nova,openstack/nova,cloudbase/nova,openstack/nova,jianghuaw/nova,vmturbo/nova,vmturbo/nova,NeCTAR-RC/nova,rajalokan/nova,NeCTAR-RC/nova,bigswitch/nova,mahak/nova,BeyondTheClouds/nova,gooddata/openstack-nova,jianghuaw/nova,sebrandon1/nova,Juniper/nova,cloudbase/nova,alaski/nova,klmitch/nova,klmitch/nova,rajalokan/nova,zhimin711/nova,BeyondTheClouds/nova,mikalstill/nova