Fix KeyError in TimeflakeBinary error handling

[Salmela]

The invalid has never apparently been supported error message in Field. The Django documentation is bit misleading as it tells that "invalid" is one of the choices, but it is not defined in the base Field class.

https://github.com/django/django/blob/517d3bb4dd17e9c51690c98d747b86a0ed8b2fbf/django/db/models/fields/__init__.py#L129

[anthonynsimon]

Thank you! There’s some unrelated errors in CI, I’ll fix those soon and merge your PR.