title: UniqueConstraint in django 2.2 don't raise ValidationError
body: <p>Since django 2.2 <a href="https://docs.djangoproject.com/en/2.2/ref/models/options/#django.db.models.Options.unique_together" rel="nofollow noreferrer">docs</a> recommend to use <code>UniqueConstraint</code> instead of <code>unique_together</code>, but this option didn't raise ValidationError in django admin without overriding <code>clean</code> or <code>validate_unique</code> methods if <code>condition</code> attribute provided.</p>
  
  <p>Is it possible to make <code>UniqueConstraint</code> raise <code>ValidationError</code>(in django admin) instead of <code>IntegrityError</code> without implementing validation logic by myself?</p>