CAMELCASE_ERRORS setting breaks __all__ field

**Note: for support questions, please use stackoverflow**. This repository's issues are reserved for feature requests and bug reports.

* **What is the current behavior?**
When `CAMELCASE_ERRORS` is set to `True` the form level all field loses it's first underscore and has a capitalized A. `_All__`

* **If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem**
Simply set `CAMELCASE_ERRORS` to `True` and trigger a form level validation error to reproduce the error.

* **What is the expected behavior?**
The field should still remain as `__all__`

* **What is the motivation / use case for changing the behavior?**
This is a bug that will cause unintended behavior.

* **Please tell us about your environment:**

- Version: 2.15.0
- Platform: Python 3.8