title: django custom form clean() raising error from clean_field()
body: <p>I have created a custom form and need to override both of the <code>clean_field()</code> method and <code>clean()</code> method. Here is my code:</p>
  
  <pre><code>class MyForm(forms.Form):
      username=forms.RegexField(regex=r'^1[34578]\d{9}$')
      code = forms.RegexField(regex=r'^\d{4}$')
  
      def clean_username(self):
          u = User.objects.filter(username=username)
          if u:
              raise forms.ValidationError('username already exist')
          return username
  
      def clean(self):
          cleaned_data = super(MyForm, self).clean()
          # How can I raise the field error here?
  </code></pre>
  
  <p>If I save this form twice, and the username will be already exist in the second time, the <code>clean_username</code> method will raise an error, however, the <code>clean()</code> method still run without interruption. </p>
  
  <p>So my question is, how can I stop calling <code>clean()</code> when error already raise by <code>cleaned_xxx</code>, if that is not possible, then how can I raised the error again which raised by <code>clean_xxxx()</code> in <code>clean()</code> method?</p>