Fix bug in error handler when response is None

Regression introduced in #298.

botocore/handlers.py

@@ -53,6 +53,10 @@ def check_for_200_error(response, operation, **kwargs):
     # 500 response (with respect to raising exceptions, retries, etc.)
     # We're connected *before* all the other retry logic handlers, so as long
     # as we switch the error code to 500, we'll retry the error as expected.
+    if response is None:
+        # A None response can happen if an exception is raised while
+        # trying to retrieve the response.  See Endpoint._get_response().
+        return
     http_response, parsed = response
     if http_response.status_code == 200:
         if 'Errors' in parsed:

tests/unit/test_handlers.py

@@ -125,6 +125,11 @@ def test_200_response_with_no_error_left_untouched(self):
         # We don't touch the status code since there are no errors present.
         self.assertEqual(http_response.status_code, 200)
 
+    def test_500_response_can_be_none(self):
+        # A 500 response can raise an exception, which means the response
+        # object is None.  We need to handle this case.
+        check_for_200_error(None, mock.Mock())
+
 
 class TestRetryHandlerOrder(BaseSessionTest):
     def get_handler_names(self, responses):