Adding retry to test_detect_web_images_from_gcs() system test.

Flaky in:
https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/1694

Author: dhermes

vision/tests/system.py

@@ -720,6 +720,7 @@ def test_detect_web_images_from_content(self):
         web_images = image.detect_web(limit=limit)
         self._assert_web_images(web_images, limit)
 
+    @RetryErrors(unittest.TestCase.failureException)
     def test_detect_web_images_from_gcs(self):
         client = Config.CLIENT
         bucket_name = Config.TEST_BUCKET.name