How to cancel save event

I have PDP and on it few Custom Fields that need to validated before project is saved. Users clicks Save button, I validate fields, if values aren't ok, Save must be rejected.

Now, I am using this solution:

http://blogs.msdn.com/b/husainzgh/archive/2011/08/01/hooking-into-the-project-detail-page-ribbon-save-button-without-overriding-out-of-box-functionality-in-project-web-access-for-project-server-2010.aspx

The problem is, I don't know how can I reject save in this solution?? Please help.