[chromedriver] Make sure build is red if there is some uncaught exception.

In the following three builds, they were still green while there were exceptions (failed to download chrome, chromedriver.exe/chromedriver_tests.exe not found, run_all_tests.py not found) and tests were not run at all.

http://build.chromium.org/p/chromium.chromedriver/builders/Win7/builds/1419

http://build.chromium.org/p/chromium.chromedriver/builders/Mac%2010.6/builds/3049

http://build.chromium.org/p/chromium.chromedriver/builders/Linux32/builds/3412

TBR=frankf
NOTRY=true

Review URL: https://codereview.chromium.org/142883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245851 0039d316-1c4b-4281-b951-d872f2087c98