title: Map on Scalaz Validation failure
body: <pre><code>import scalaz._
  import Scalaz._
  
  "abc".parseInt
  </code></pre>
  
  <p>This will return a <code>Validation[NumberFormatException, Int]</code>.
  Is there a way I can apply a function on the failure side (such as <code>toString</code>) to get a <code>Validation[String, Int]</code>?</p>