title: Django Import - Export: IntegrittyError when trying to insert duplicate record in field(s) with unique or unique_together constraints
body: <p><strong>Update</strong></p>
  
  <p>I have filed a <a href="https://github.com/django-import-export/django-import-export/issues/382" rel="noreferrer">feature request</a>. The idea is to <code>pass</code> on the <code>IntegrittyError</code> produced by the database when <code>unique</code> or <code>unique_together</code> reject a record that already exists in the database.</p>
  
  <hr>
  
  <p>I have the following model:</p>
  
  <pre><code>class Compositions(models.Model):
      composer_key = models.ForeignKey(
          Composer,
          )
      composition = models.CharField(
          max_length=383,
          )
  
      class Meta(object):
          unique_together = (('composer_key', 'composition'), )
  </code></pre>
  
  <p>Using django-import-export in the admin interface, <strong>without providing an <code>id</code> for each entry in the csv file</strong>,  ... if one pair of the csv file already exists, the procedure will be interrupted with an integrity error</p>
  
  <pre><code>duplicate key value violates unique constraint "data_compositions_composer_key_id_12f91ce7dbac16bf_uniq"
  DETAIL:  Key (composer_key_id, composition)=(2, Star Wars) already exists.
  </code></pre>
  
  <p>The CSV file is the following:</p>
  
  <pre><code>id  composer_key    composition
          1           Hot Stuff
          2           Star Wars
  </code></pre>
  
  <p><a href="https://github.com/django-import-export/django-import-export/issues/382#issuecomment-170454385" rel="noreferrer">The idea</a> was to use <code>skip_row</code> and implement it in the admin.</p>
  
  <p>admin.py:</p>
  
  <pre><code>class CompositionsResource(resources.ModelResource):
  
      class Meta:
          model = Compositions
          skip_unchanged = True
          report_skipped = True
  
  
  class CompositionsAdmin(ImportExportModelAdmin):
      resource_class = CompositionsResource
  
  
  admin.site.register(Compositions, CompositionsAdmin)
  </code></pre>
  
  <p>This will not cure the problem, however, because <code>skip_row</code> expects an <code>id</code> in the csv file in order to check if each row is the same with the very specific database entry. </p>
  
  <p>Considering that this control can be performed by the database when using <code>unique</code>(<code>_together</code>) would not it be effective to catch this error and then return <code>skip_row = True</code> or alternatively <code>pass</code> on this error?</p>