title: extjs 4 - How change default message error in field
body: <p>I have a form panel in <b><a href="http://jsfiddle.net/7CLWy/" rel="noreferrer">http://jsfiddle.net/7CLWy/</a></b>
  here is my important code </p>
  
  <pre><code>items: [{
      xtype: 'textfield',
      fieldLabel: 'First Name',
      allowBlank: false,
      msgTarget: 'under',
      name: 'firstName'
  }, {
      xtype: 'datefield',
      allowBlank: false,
      fieldLabel: 'Start date',
      msgTarget: 'under'
  }],
  </code></pre>
  
  <p>I want change default message error in field </p>
  
  <p><img src="https://i.stack.imgur.com/pAnU4.jpg" alt="enter image description here"></p>
  
  <p>How to change that. Thanks</p>