Handle 'test error' to deal with error response from invalid browsers.

Tested with configuration:

      'saucelabs-custom': {
        all: {
          options: {
            urls: [
              'http://127.0.0.1:3000/'
            ],
            browsers: [
              {
                browserName : 'chrome',
                platform : 'OS X 10.11',
                version : 26 
              }
            ],
            statusCheckAttempts : -1
          }
        }
      }

Chrome 26 on OS X 10.11 is unsupported by sauce labs. Returns a response where result.status is "test error". Expected behavior: test fails with an error. Actual behavior: request will be re-attempted, and when statusCheckAttempts is set to -1, the test will never complete.

This PR causes the test to complete with an error.

axemclion / grunt-saucelabs

Handle 'test error' to deal with error response from invalid browsers. #225