When no user/group was found, migration plugin reported an ambiguous
error about invalid container. But the root cause may be for example
in a wrong list of user/group objectclasses. Report both in the error
message to avoid user confusion.
User/group objectclass attribute is now also marked as required.
Without the list of objectclasses, an invalid LDAP search is
produced.
https://fedorahosted.org/freeipa/ticket/2206