TC runs can be successful when throwing Python exceptions

[gsnedders]

e.g. see https://tools.taskcluster.net/groups/JWdij6zCR4mWmw2V4ogQXA/tasks/Ll6F-MrBSOyvAtEG7viEgg/runs/0/logs/public%2Flogs%2Flive.log

INFO:manifest:Manifest downloaded
Traceback (most recent call last):
  File "./wpt", line 5, in <module>
    wpt.main()
  File "/home/test/web-platform-tests/tools/wpt/wpt.py", line 132, in main
    rv = script(*args, **kwargs)
  File "/home/test/web-platform-tests/tools/wpt/run.py", line 490, in run
    **kwargs)
  File "/home/test/web-platform-tests/tools/wpt/run.py", line 459, in setup_wptrunner
    channel = install.get_channel(kwargs["product"], kwargs["channel"])
  File "/home/test/web-platform-tests/tools/wpt/install.py", line 49, in get_channel
    channel = channel[browser]
KeyError: 'chrome_webdriver'
[taskcluster 2018-09-06 16:30:36.701Z] === Task Finished ===
[taskcluster 2018-09-06 16:30:38.432Z] Successful task run with exit code: 0 completed in 88.828 seconds

This shouldn't be a success.