vcbuild explodes is Python is not installed

seishun

Version: master
Platform: Windows 7
Subsystem: build

If Python is not installed on the system, vcbuild prints out the following and creates a "Windows can't open this file" dialog

ERROR: The system was unable to find the specified registry key or value.
ERROR: The system was unable to find the specified registry key or value.
Cannot determine current version of Node.js

cc @refack