Have the test suite check error messages.
Since XMLRPC doesn't give us structured error information, just
compare the resulting text.
Fix messages that tests expect to cause.
Minor changes:
Make netgroup-mod's NotFound message consistent with other objects
and methods.
In test_automember_plugin, test with nonexistent automember rules
of both types, instead of nonexistent users.
https://fedorahosted.org/freeipa/ticket/2549