title: Extjs 4 rowediting custom validation
body: <p>using plugin RowEditing on a grid  how to display a custom Error Message on the  'validateedit'  when i cancel the validation ?</p>
  
  <pre><code>validateedit :  function(editor, e) {
  
       if (condition) {
      e.cancel = true;
      // how to add an error message to a field 
      }
  }
  </code></pre>