[HOTFIX] Add vtkmodules/web/errors.py

Resolves #18767 (closed) by following up to !9727 (merged) to add vtkmodules/web/errors.py to the wheel.

@ben.boeckel, please review. I'm surprised the CI testing didn't pick up on this. We need to look into that after this merges

---

Backport: paraview/release
Backport: release