Propagate original HTTP error event

[liy]

There could multiple types of loading error. At the moment it is hidden by the custom ErrorEvent. For example, there is no way to tell the file request is cancelled or it is actually missing.

https://github.com/CreateJS/PreloadJS/blob/master/src/preloadjs/LoadQueue.js#L1611

It make sense to keep the original http error code in your custom ErrorEvent.