fix: changed AssImp unsupported pivot bone error to warning

[Doprez]

PR Details

Some projects fail to build due to a possible regression in the AssImp importer from FBX. The failure is due to FBX files being given virtual nodes that describe translation, rotation and scale in the form of $AssimpFbx$ nodes. This changes the error to be a warning as it seems to be importing correctly and animating correctly.

These were 3 previously problematic models that seem to be working fine:

Related Issue

#2459 - technically a fix for the reported issue but not for the actual cause.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.

[Eideren]

Thanks !