These changes make the execution abort earlier on an error (like a tool is not found) and makes it easier to figure out what's wrong.
Made build_zxing.py executable.
BUG=None
TEST=Local runs of the PyAuto test src/chrome/test/functional/webrtc_video_quality.py in Chromium.
Review URL: https://webrtc-codereview.appspot.com/840005
git-svn-id: http://webrtc.googlecode.com/svn/trunk@2899 4adac7df-926f-26a2-2b94-8c16560cd09d