title: Form.non_fields_errors()
body: <p>I know how to display a 'bound' message on a form with </p>
  
  <pre><code>def clean_ref1_fullname(self):
      value = self.form.cleaned_data.get('ref1_fullname', '').title()
      if value in (None, ''):
          raise ValidationError(
              _('"Reference 1 - Fullname" is required')
          )
      return value
  
  def clean_ref1_fullname(self):
      value = self.form.cleaned_data.get('ref1_fullname', '').title()
      if value in (None, ''):
          raise ValidationError(
              _('"Reference 1 - Fullname" is required')
          )
      return value
  </code></pre>
  
  <p>It will display a message on a specific field. I would like to display a global message. How could I do that? Assume this form is simply <code>ExampleForm()</code>.</p>