Retry 400 Bad errors

[bbangert]

For 'reasons', we sometimes get a flakey 400 Bad Request for a security token. These requests should be retried or return a 503, and ping a metric so we know they still happen. Logging to Sentry isn't needed though.

JSONResponseError: JSONResponseError: 400 Bad Request
{u'message': u'The security token included in the request is invalid.', u'__type': u'com.amazon.coral.service#UnrecognizedClientException'}
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...
  File "boto/dynamodb2/table.py", line 913, in delete_item
    conditional_operator=conditional_operator)
  File "boto/dynamodb2/layer1.py", line 925, in delete_item
    body=json.dumps(params))
  File "boto/dynamodb2/layer1.py", line 2842, in make_request
    retry_handler=self._retry_handler)
  File "boto/connection.py", line 954, in _mexe
    status = retry_handler(response, i, next_sleep)
  File "boto/dynamodb2/layer1.py", line 2885, in _retry_handler
    data)