Clarify how loaders should communicate errors

[binki]

I was looking at https://github.com/amdjs/amdjs-api/blob/67486d6b199cd026f5e66151566836c8ba149099/LoaderPlugins.md and couldn’t find anything obviously sticking out on how a loader plugin should communicate load failure. It doesn’t seem like load() should be called when there is an error. Should there be a new error callback or something?

[jrburke]

I don't think there is much interest in fleshing out the loader plugin API much now, so I would not expect much activity here. requirejs does allow an error notification by calling load.error(), described as onload.error in its plugin doc section.