notification of error in loaders, importers

related to issue #16

Hi,

The request is not completed (yet), I did a first pull request to start discussion on the API. (before changing every loader, importer and test).

I inject AssetPackTrace as argument for load and import (and as field of AssetManager). Are you ok ?

Currently I continue to think that load, import should use Future.error to report error and only AssetManager should take care of AssetPackTrace and update it (push events into) and may be convert load error into payload = null for importer (it is strange to import after a failing load). as is load and import api follow Future usage, api is simple and recovery rules can be managed by AssetManager.