ExpandVar doesn't print the input file name on error
ExpandVar doesn't print the input file name on error

Make the error more informative.

Patch contributed by Florent Thoumie
BUG=45

Review URL: http://codereview.appspot.com/88147

old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google