title: Validation (with custom ErrorTemplate) for a DataTemplate
body: <p>I can't figure this guy out / in desperate need of assistance.</p>
  
  <p>I have an ItemsControl and a DataTemplate as the ItemTemplate</p>
  
  <p>i.e.</p>
  
  <pre><code>&lt;DataTemplate&gt;
      &lt;StackPanel&gt;
         &lt;TextBox Text={Binding Prop1}/&gt;
         &lt;TextBox Text={Binding Prop2}/&gt;
      &lt;/StackPanel&gt;
  &lt;/DataTemplate&gt;
  </code></pre>
  
  <p>I have seen plenty of examples for applying validation to the target of an individual binding (i.e. just validating each textbox separately).  I'd like to have a visual change to the whole DataTemplate if there is ANYTHING wrong (either Prop1 OR Prop2), using an <code>&lt;AdornedelementPlaceholder/&gt;</code>, as I've seen in many examples.</p>
  
  <p>Essenetially, is there a way to show an error if ANY item in the object that represents my DataContext of the DataTemplate has a problem?</p>