While not validating models, still need a way to delete downloaded models that aren't valid

[phildow]

the model manager can only list model models that have loaded, and if the model.json is bad, the bundle won't load, so there will be invisible models sitting on the filesystem if the user downloaded them.

only necessary if a fix for #83 doesn't come quickly, since validation will prevent a model from ever hitting the filesystem

[phildow]

For now just try to load the model bundle and consider that validation. Let the user know the problem is either with the packaging or the model.json file without more detail