title: Set Spark TextInput Background when fail validation
body: <p>How do I specify what a TextInput looks like in the error state? There appears to be a real scarcity of documentation on the subject!</p>
  
  <p>There isn't an error state, but there is an errorSkin which you can specify (does it have to be a subclass of ErrorSkin?). </p>
  
  <p>I want to set the TextInput's background color and increase the thickness of the border when validation fails. How do I do this?</p>