Support custom template properties in ValidationFailure.render

The `Form.render` method takes additional arguments, which allows to pass custom properties to the template.

E.g.:

    rendered = form.render(obj_dict, custom_property="My data")

Then in the template `form.pt` you can access this property:

    <p>${custom_property}</p>

Unfortunately the `render` method of `ValidationFailure` does not allow to pass additional arguments, so that you can not pass custom properties to the template. The PR fixes this.