Exception handling when asset bundle has not completed loading

[dalkia]

AssetBundles are loading in an async manner, so if the file is valid but the asset bundle is not, the loading process does not fail.

This has been fired in this thread, and a solution has been proposed on this PR.

We need to validate it before merging it.

This could be done locally by creating some failed asset bundles and trying to load them to see what occurs