Better control of unprocessed item response to batch requests

[rclark]

In the event that a batch request returns unprocessed items, this PR improves cardboard's error response.

If this occurs, the err.unprocessed will be:

• a feature collection in the case of cardboard.batch.put
• an array of ids in the case of cardboard.batch.remove

This can be used to retry the request, if appropriate.