title: Model validation values in custom Django form error messages
body: <p>I have created a Django form: </p>
  
  <pre><code>class AddFaceForm(forms.ModelForm):
  
      class Meta:
          model = Face
          fields = ('person', 'x', 'y', 'w', 'h')
  </code></pre>
  
  <p>In the <code>Face</code> model, <code>w</code> is defined as </p>
  
  <pre><code>w = models.DecimalField(
      'Relative width',
      validators=[MinValueValidator(0.05), MaxValueValidator(1)]
  )
  </code></pre>
  
  <p>If a user enters 0 for the width, the error message Django returns upon form validation is </p>
  
  <blockquote>
    <p>Ensure this value is greater than or equal to 0.05.</p>
  </blockquote>
  
  <p>So it nicely contains the value from the <code>MinValueValidator</code>. I would like to change it to "Relative width must be >= 0.05".</p>
  
  <p>To achieve it, I tried customising errors in AddFaceForm Meta</p>
  
  <pre><code>class AddFaceForm(forms.ModelForm):
  
      class Meta:
          model = Face
          fields = ('person', 'x', 'y', 'w', 'h')
          error_messages = {
              'w': {
                  'min_value': 'Relative width must be &gt;= 0.05'
              }
          }
  </code></pre>
  
  <p>But this means hard-coding the 0.05 value. I would like to read it from the model definition. How can this be done?</p>