change error message for not selecting a template

needed to subclass RadioField for this

tests/app/main/views/test_template_folders.py

@@ -1092,3 +1092,27 @@ def test_should_be_able_to_move_to_new_folder(
         folder_ids={FOLDER_TWO_ID},
         template_ids={TEMPLATE_ONE_ID},
     )
+
+
+def test_radio_button_with_no_value_shows_custom_error_message(
+    client_request,
+    service_one,
+    mock_get_service_templates,
+    mock_get_template_folders,
+    mock_move_to_template_folder,
+    mock_create_template_folder,
+):
+    service_one['permissions'] += ['edit_folders']
+
+    page = client_request.post(
+        'main.choose_template',
+        service_id=SERVICE_ONE_ID,
+        _data={'operation': 'add-new-template'},
+        _expected_status=200,
+        _expected_redirect=None,
+    )
+
+    assert mock_move_to_template_folder.called is False
+    assert mock_create_template_folder.called is False
+
+    assert page.select_one('span.error-message').text.strip() == 'Select the type of template you want to add'