Fixed #16986 -- Model.clean() can report errors on individual fields.

This commit provides the tests for this issue but the actual problem was solved
by the ValidationError refactor in f34cfec and ee77d4b.

Refs #20199.

tests/model_forms/tests.py

@@ -1782,6 +1782,14 @@ class OldFormForXTests(TestCase):
             '<ul class="errorlist"><li>Model custom error message.</li></ul>'
         )
 
+    def test_model_clean_error_messages(self) :
+        data = {'name1': 'FORBIDDEN_VALUE', 'name2': 'ABC'}
+        errors = CustomErrorMessageForm(data).errors
+        self.assertHTMLEqual(
+            str(errors['name1']),
+            '<ul class="errorlist"><li>Model.clean() error messages.</li></ul>'
+        )
+
 
 class M2mHelpTextTest(TestCase):
     """Tests for ticket #9321."""