Fix expected error messages in tests

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

tests/test_xmlrpc/test_selfservice_plugin.py

@@ -39,21 +39,24 @@ class test_selfservice(Declarative):
         dict(
             desc='Try to retrieve non-existent %r' % selfservice1,
             command=('selfservice_show', [selfservice1], {}),
-            expected=errors.NotFound(reason='no such entry'),
+            expected=errors.NotFound(
+                reason=u'ACI with name "%s" not found' % selfservice1),
         ),
 
 
         dict(
             desc='Try to update non-existent %r' % selfservice1,
             command=('selfservice_mod', [selfservice1], dict(description=u'Foo')),
-            expected=errors.NotFound(reason='no such entry'),
+            expected=errors.NotFound(
+                reason=u'ACI with name "%s" not found' % selfservice1),
         ),
 
 
         dict(
             desc='Try to delete non-existent %r' % selfservice1,
             command=('selfservice_del', [selfservice1], {}),
-            expected=errors.NotFound(reason='no such entry'),
+            expected=errors.NotFound(
+                reason=u'ACI with name "%s" not found' % selfservice1),
         ),