Wrap rebaseline-o-matic in a try/except.

If the git pull or webkit-patch commands error, then rebaseline-o-matic
should keep trying instead of dieing.

Review URL: https://chromiumcodereview.appspot.com/22779002

git-svn-id: svn://svn.chromium.org/blink/trunk@155989 bbb929c8-8fbe-4397-9dbb-9b2b20218538