StorageManager#loadModel and #_handleNextLayer with error callbacks

StorageManager#loadModel will invoke an error callback if the result set  is empty
* The error callback is invoked (or silently breaks) when the server responds with 0 results.

A couple question for when the review process happens:

* What should the arguments be for the error callback? 
* Are we going to provide a default error callback? 


StorageManager#_handleNextLayer will pass the error callback to additional server requests.