Make run-webkit-tests resilient to errors from wdiff().

Right now if wdiff fails with an error, it crashes run-webkit-tests,
which is overly harsh :). Change things to log the error and mark
wdiff as not available, so that we can keep going.

BUG=255145
R=ojan@chromium.org, iannucci@chromium.org, eseidel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19601008

git-svn-id: svn://svn.chromium.org/blink/trunk@154718 bbb929c8-8fbe-4397-9dbb-9b2b20218538