Server errors can result in duplicate responses #180
Description
When we receive a 500 from the API, we go into offline mode. That can happen when the server chokes on an awkward geometry (like at 0 Water St, Toledo, OH). It looks the subsequent, simple responses are getting properly processed by the server, but the 500 response means the client will retry them anyway.
The simple way out is to submit responses one at a time when we exit offline mode. Then we can react to errors for specific responses.