title: IValueConverter and validation on exception
body: <p>I'm using custom DateTimeToString : IValueConverter</p>
  
  <p>In my ConvertBack method I'm throwing Exception when conversion fails, however it is not displayed as validation failure (it is an unhandled application exception), and I want to show it as validation problem (red border).</p>
  
  <p>In short I want it to work like DateTime+Texbox when it shows validation message ("input string was in incorrect format") but with my custom IValueConverter.</p>