failover: proceed normally when no new server is found
    
    Multiple failover requests come in same time, the first one will
    result in collapsing the meta server but multiple resolution of
    SRV records are triggered. The first one finishes normally but the
    others won't find any new server thus ends with an error.
    
    This patch makes failover to proceed normally even in such case.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3131
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>