Validation of the fields argument to bulk_update

[UnknownPlatypus]

I have made things!

Mostly mirrors validation logic from django.db.models.query.QuerySet.bulk_update.
Tests are inspired from django/tests/queries/test_bulk_update.py

I could have called a dummy bulk_update with no objects to reuse django internal logic but it felt brittle with risk of someday actually do database query so I preferred to re-implement the few checks

[sobolevn]

Please, also test at least one nested__field error. It is not supported by django.

[sobolevn]

Great work!