Bug: re-consider some scene validation errors as warnings

I would suggest report this as a warning instead of error:

https://github.com/assimp/assimp/blob/59bc03d931270b6354690512d0c881eec8b97678/code/PostProcessing/ValidateDataStructure.cpp#L420

Although having two bones with the same name is likely a problem, we might still want to load the model. For instance we might not need animation at all, or know how to handle such scenarios for this particular model. Finally we might use the first bone we found with that name.

Comparing to another validation errors like out of range indices, this one doesn't look that bad.

This applies to assimp 6.0.2 and the current master.

Start in code/PostProcessing/ValidateDataStructure.cpp at line 420 and inspect how the duplicate-bone-name validation is reported, along with nearby validation cases. Confirm the expected behavior for models with duplicate bone names and verify that this condition is reported as a warning while the model still loads.

Written by the indexing model from the issue text.