Catch WebDriver exceptions when creating selenium browser

Observed this exception twice in the cluster:

    selenium.common.exceptions.WebDriverException: Message: u'chrome not reachable\n  (Driver info: chromedriver=2.1,platform=Linux 3.2.0-40-virtual x86_64)' 

I was able to reproduce locally in about 1/50 trials.  This change appears to have resolved this issue (ran 200 trials successfully).

@jzoldak