TST: Asserts all types are caterogical in function fillna with correct error message
- [x] closes #13628
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff
- [x] whatsnew entry

needs `isort pandas/tests/arrays/categorical/test_missing.py` thanks @LiuSeeker 