I'm using WAMPServer V2.5 (apache 2.4.9, PHP 5.6.17) while testing BigTree and so far things have been going well.  I just clicked the upgrade to 4.2 button and the upgrade failed with the following errors:

Warning: include(../core/admin/router.php): failed to open stream: No such file or directory in C:\wamp\www\bigtree\site\index.php on line 56

Warning: include(): Failed opening '../core/admin/router.php' for inclusion (include_path='.;C:\php\pear') in C:\wamp\www\bigtree\site\index.php on line 56

What appears to be happening is the core directory gets backed up to backups directory, but this error is thrown before the new updated core directory is copied, so of course the files in /core/admin aren't found.

Anyone have any ideas why this may be breaking, and what I can do as a workaround?  I've was able to simply extract the core directory from the install file to the appropriate place and things seem to work, but I'm wondering if the install script is failing before other things in the upgrade process have a chance to happen? 
Is there another way to upgrade besides using the Upgrade button?