BUG/TST:  fillna limit checks and tests
Reversed the order of returning and performing checks on `limit`
in fillna method in blocks.py so that an ValueError is raised
before returning.

Alternative to PR #27074


- [tick ] closes #27042 
- [ ] tests added / passed
- [tick ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ tick] whatsnew entry
