Magento 1.4 upgrade - Missing Yesnocustom.php
After upgrade from 1.3.2.4 to 1.4.0.1 using MagentoConnect we got exception in Admin/System page.
After checks we have figured out that the file was missing in upgrade package for Magento Core Modules.
File was used by system.xml located in Mage/Core/etc/.
In order to fix this you have to download Magento package 1.4.0.1 from website and add missing file to your sources
app/code/core/Mage/Adminhtml/Model/System/Config/Source/Yesnocustom.php