title: Customize Maya's addCheckCallback pop up message
body: <p>When the user saves a file I want a check to happen prior to saving. If the check fails then it doesn't save. I got this working with mSceneMessage and kBeforeSaveCheck, but I don't know how to customize the pop-up message when it fails. Is this possible?</p>
  
  <pre><code>import maya.OpenMaya as om
  import maya.cmds as cmds
  
  def func(retCode, clientData):
      objExist = cmds.objExists('pSphere1')
      om.MScriptUtil.setBool(retCode, (not objExist) ) # Cancel save if there's pSphere1 in the scene
  
  cb_id = om.MSceneMessage.addCheckCallback(om.MSceneMessage.kBeforeSaveCheck, func)
  </code></pre>
  
  <p>Right now it displays</p>
  
  <blockquote>
    <p>File operation cancelled by user supplied callback.</p>
  </blockquote>