failover: return error when SRV lookup returned only duplicates
    
    https://fedorahosted.org/sssd/ticket/1947
    
    Otherwise we risk that the meta server is removed from the server list,
    but without a chance to return, because there may be no fo_server with
    srv_data = meta.
    
    Also if state->meta->next is NULL (it is still orphaned because we try
    to errornously expand it without invoking collapse first), state->out
    will be NULL and SSSD will crash.
    
    New error code: ERR_SRV_DUPLICATES