report mocha test failures to correct test strategy

This grunt task will never fail (process exit with non-zero) if mocha test fails. which breaks the actual usage in a CI environment. For instance, command `$ grunt mochaWebdriver` will always exit with status 0. which is unexpected.

Then I found it never touches `err` parameter inside the function `onTestFinish`, which means mocha test errors are ignored. so I just simply report the mocha test result out.

I didn't provide parameter for callback function of `quit` because it kills phantom in anyway or shows "One or more tests on (Selenium|Sauce Labs) failed.". which should be caused by mocha test failure(s).