Improve asset file errors

Issues like using a class name that does not exist in a sub object or not putting quotes around a identity i.e. m_Components: [2] or m_Components: 2 cause crashes that are difficult to understand. Would be much better to have good error messages.

Handle index out-of-range.

I was hitting this recently as well. Many crash vectors seem to be specifying incorrect paths for assets. Also, things don't go well when trying to put a json asset in the data root (no package).