Make error pointers work with key transformations.

[stas]

What is the current behavior?

Error pointers are blank when using set_key_transform in the serializer.

What is the new behavior?

Fixes #56

Checklist

Please make sure the following requirements are complete:

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [x] All automated checks pass (CI/CD)

[stas]

@bsylvain this little patch should fix the issue. Thanks for raising the issue!