Fix bug: continue processing model if skeleton check fails

[tellypresence]

There's a problem in src/tydra/render-data.cc around line 3704. If the skeleton check fails, the model processing is aborted at that point, instead of continuing with other crucial model processing steps...

  if (skelPath.is_valid()) {
    SkelHierarchy skel;
    nonstd::optional<Animation> anim;
    if (!ConvertSkeletonImpl(env, mesh, &skel, &anim)) {
      return false; // << Early abort doesn't continue processing model
    }
    // Process skeleton
  }
  // Additional model processing from here never happens if skeleton check fails
  //
  // 6. BlendShapes
  //
...
  //
  // 7. Compute normals
  //
...
  //
  // 8. Build indices
  //

If the logic is inverted and early (aborting) return removed, the model processing is allowed to continue even if the skeleton check fails

  if (skelPath.is_valid()) {
    SkelHierarchy skel;
    nonstd::optional<Animation> anim;
    if (ConvertSkeletonImpl(env, mesh, &skel, &anim)) {
      // Process skeleton
    }
  ...
  }
  // Additional model processing happens here regardless of status of skeleton check
  //
  // 6. BlendShapes
  //
...
  //
  // 7. Compute normals
  //
...
  //
  // 8. Build indices
  //

Additional details in #176

[syoyo]

If Skeleton exits, but failed to parse Skeleton, it should report error at the moment.

Also you should test a scene with minimal&reproducible manner and using TinyUSDZ tool solely, not with assimp.