Add error message on metadata API
Add error message on metadata API

The changed BadRequest message didn't explain why that is wrong to
API consumers. This patch adds it.

Change-Id: Idbd183f13045c85485dfa428974511feb6e407f7

mikalstill/nova,mahak/nova,mikalstill/nova,gooddata/openstack-nova,mikalstill/nova,phenoxim/nova,rahulunair/nova,gooddata/openstack-nova,klmitch/nova,klmitch/nova,gooddata/openstack-nova,mahak/nova,openstack/nova,rahulunair/nova,openstack/nova,phenoxim/nova,gooddata/openstack-nova,klmitch/nova,mahak/nova,rahulunair/nova,klmitch/nova,openstack/nova