title: Validation.HasError does not trigger again if new error comes in while already true
body: <p>I use MVVM and my object implement IDataErrorInfo.  When a property is set, I run custom validation methods and if the validation passes, I return String.empty, which sets Validation.HasError to false.  If the validation fails, Validation.HasError is set to true.  I have a style that I use for "required controls" (controls that will perform the validation) and set's the ToolTip of the control to whatever the error is like this:</p>
  
  <pre><code>&lt;Style x:Key="RequiredControl" TargetType="{x:Type Control}" &gt;
      &lt;Style.Triggers&gt;
          &lt;Trigger Property="Validation.HasError" Value="true"&gt;
              &lt;Setter Property="ToolTip" Value="{Binding (Validation.Errors), Converter={StaticResource ErrorConverter}, RelativeSource={x:Static RelativeSource.Self}}"/&gt;
          &lt;/Trigger&gt;
      &lt;/Style.Triggers&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>And the ErrorConverter:</p>
  
  <pre><code>public class ZynErrorContentConverter : IValueConverter
  {
      public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
      {
          var errors = value as ReadOnlyObservableCollection&lt;ValidationError&gt;;
          if (errors == null) return "";
  
          return errors.Count &gt; 0 ? errors[0].ErrorContent : "";
      }
  
      public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
      {
          throw new NotImplementedException();
      }
  }
  </code></pre>
  
  <p>The problem is this: The user enters something invalid...and the Validation.HasError is set to true.  The tooltip updates as it is supposed to.  If the user attempts to correct the error, but enters a value that causes a different type of invalidation, the tooltip should show the new error string, but this doesn't happen.  The error shows as the same error from the first error.  I know why this happens (I think)...Because the Trigger is not triggered because the Validation.HasError never changes from True -> False -> True.</p>
  
  <p>Does anyone have any experience with this or some advice as to how to force the trigger?</p>