baseldap: Handle missing parent objects properly in *-find commands
    
    The find_entries function in ipaldap does not differentiate between
    a LDAP search that returns error code 32 (No such object) and LDAP
    search returning error code 0 (Success), but returning no results.
    
    In both cases errors.NotFound is raised. In turn, LDAPSearch
    commands interpret NotFound exception as no results.
    
    To differentiate between the cases, a new error EmptyResult
    was added, which inherits from NotFound to preserve the compatibility
    with the new code.
    
    This error is raised by ipaldap.find_entries in case it is performing
    a search with and the target dn does not exist.
    
    https://fedorahosted.org/freeipa/ticket/4659
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>