Retry whole batch of deletes on 429.

Because 'Bucket.delete' inside a batch context manager is deferred,
we cannot retry it directly.  We also need to check for bucket existence,
as the bucket *may* have been deleted in the previous batch before the
429 was raised.

Closes #3761.

storage/tests/system.py

@@ -82,11 +82,15 @@ class TestStorageBuckets(unittest.TestCase):
     def setUp(self):
         self.case_buckets_to_delete = []
 
-    def tearDown(self):
+    def _delete_buckets(self):
         with Config.CLIENT.batch():
             for bucket_name in self.case_buckets_to_delete:
                 bucket = Config.CLIENT.bucket(bucket_name)
-                retry_429(bucket.delete)()
+                if bucket.exists():
+                    bucket.delete()
+
+    def tearDown(self):
+        retry_429(self._delete_buckets)()
 
     def test_create_bucket(self):
         new_bucket_name = 'a-new-bucket' + unique_resource_id('-')