BUG: .reset_index() should raise with an invalid level name (GH20925)
#20925 Raises appropriate error for Series.reset_index(level_name, drop=True) when index is flat and an invalid level is supplied

- [x] closes #20925 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry