Fix exception message formating
Fix exception message formating

Correct a trivial message formatting mistake.

Change-Id: Ic011b4aad6d30cf97c94e3dbe48924fbbbf7f836

stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2,stackforge/networking-bagpipe-l2