Improve tracing of xvfb.py

Route stdout to stdout.
Report error code.
Reword failure to test message.

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/385073006

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@283022 4ff67af0-8c30-449e-8e8b-ad334ec8d88c