[autotest] Fix error message in autotest.py

TEST=Threw and exception.
BUG=None

Change-Id: I548d1afb936ad9f6e6e2580833c47f9dbb1f4aa6
Reviewed-on: https://chromium-review.googlesource.com/170569
Tested-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Prashanth B <beeps@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
1 file changed