Object: Give more helpful error message in TestServiceVersion
Object: Give more helpful error message in TestServiceVersion

It is very helpful when we bump service rpc version since we have
test_version() case to cover that, but sometime we do rebase/bump
rpc service version we may forget to increae compute_manager's version,
in this case test_version give a wrong message will make `new` developer
confused, and spend time to do debuging, this patch improves the error
message.

Change-Id: I64320b5a46aa0543e3d9e8fdd27f6faa3829cf69

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