title: Django: custom validation message
body: <p>I'm having a problem with changing my validation message from
  'Please fill out this field' to 'Custom message blablabla':
  I tried to use something like this:</p>
  
  <pre><code>class NameForm(ModelForm):
      def __init__(self, *args, **kwargs):
            super(NameForm, self).__init__(*args, **kwargs)
            self.fields['name'].error_messages['required'] = 'Testing'
  
      class Meta:
          model = Player
          fields = ['name']
          widgets = {'name': forms.TextInput(
                       attrs={'id': 'player_name', 'required': True, 'placeholder': 'Player name...','value':'player1','title':'write here your name'}
              )
          }
  </code></pre>
  
  <p>but it doesn't seem to work. What am I missing? How can I customize my error message for a specific field when required is raised?</p>