Give more meaningful error message when no servers are configured
Give more meaningful error message when no servers are configured

If the user tries to create a domain and there are no servers
configured, the error message is 'configuration_error.' This
can be confusing, especially for new users.

This changes the message to be 'no_servers_configured'

Change-Id: Ic02d1f516db659a0c09838729c51ac386280f7eb
Fixes: bug#1204578

richm/designate,grahamhayes/designate,openstack/designate,cneill/designate-testing,tonyli71/designate,kiall/designate-py3,tonyli71/designate,ionrock/designate,tonyli71/designate,cneill/designate,NeCTAR-RC/designate,cneill/designate,cneill/designate-testing,kiall/designate-py3,kiall/designate-py3,ramsateesh/designate,muraliselva10/designate,richm/designate,ramsateesh/designate,kiall/designate-py3,melodous/designate,cneill/designate,muraliselva10/designate,ionrock/designate,grahamhayes/designate,cneill/designate,melodous/designate,cneill/designate,melodous/designate,openstack/designate,melodous/designate,muraliselva10/designate,openstack/designate,grahamhayes/designate,ionrock/designate,ramsateesh/designate,cneill/designate-testing,kiall/designate-py3,NeCTAR-RC/designate