It might be overcomplicating things, but would it be useful to add in a callback for the `onEach`? 
I'd also like to split out the onEach functionality and the completion/error handler into 2 callbacks.

```javascript
SearchAPI.registerReindexAllHandler('content', function(callback) {
    var onEach = function(rows, onEachCallback) {
        // Possibly do something async with rows and finally call the callback.
        onEachCallback(null);
    };
    ContentDAO.iterateAll(['contentId'], 1000, onEach, callback);
    
```