Stageusedr-activate: show username instead of DN

If activate user already exists, show name of this user in error message
instead of user DN.
Error message reworder to keep the same format as stageuser-add,
user-add.

https://fedorahosted.org/freeipa/ticket/5038

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/plugins/stageuser.py

@@ -682,8 +682,9 @@ class stageuser_activate(LDAPQuery):
                 active_dn, ['dn']
             )
             assert isinstance(staging_dn, DN)
-            raise errors.DuplicateEntry(message=_('Active user %(user)s already exists') % dict(
-                            user=test_entry_attrs.dn))
+            raise errors.DuplicateEntry(
+                message=_('active user with name "%(user)s" already exists') %
+                dict(user=args[-1]))
         except errors.NotFound:
             pass