Enhancement: retrieving unsupported/unknown extensions used in an asset

[stripe2933]

It would be awesome if we can retrieve the unsupported (not included in Parser constructor's extensionsToLoad parameter) or unknown (from fastgltf) extension.

One idea is: add extensionUnsupported and extensionUnknown public fields to Parser and modifying them when load{Gltf,GltfJson,GltfBinary} called.

[spnda]

After the next release I was going to re-design the entire error handling to (1) include more error codes and (2) for important/common errors include error messages, similar to std::exception::what. I think that would also solve this problem.

[stripe2933]

Looking forward it. Thanks!

[spnda]

I'll keep this open just as a reminder and so that people can see when this functionality is added.