Simplify OpenSeadragon.makeAjaxRequest

* Since async is always true – and browsers are starting to deprecate
  synchronous XHR – we were able to prune considerable amount of code
* Add an error callback to match the existing success callback
Better match project style
makeAjaxRequest: log all errors, test callback

* All AJAX errors will log to the console
* The onError callback will only be called if defined
makeAjaxRequest: throw error for missing callback