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