Improve dnszone-add error message
    
    When a new DNS record is being added to DNS zone via command
      ipa dnsrecord-add ZONE @
    and the target ZONE does not exist it returns ObjectclassViolation
    which may confuse users. Make sure that standard DNS Zone NotFound
    exception is returned.
    
    https://fedorahosted.org/freeipa/ticket/2270