Improve addJSON-related error handling

In addJSON itself, we should remove the full body logging on an encode error. It doesn't provide any particular benefit, given that the JSON being logged has either already been proven to be valid (it's from the bucket), or is something created by Sync Gateway.

In addition, several callers of addJSON are ignoring errors. In the event of an error that's caused by client disconnection, it makes sense to detect the error and stop iterating over other results. Right now addJSON is only checking for "write: broken pipe" - it would be a good idea to identify a broader set of 'client disconnect' type errors and handle them in the same way.