BUG: pd.Series.interpolate non-numeric index column (21662)
- [x] closes #21662
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Raises a helpful exception when a non-numeric index is sent to ```interpolate``` with methods which require numeric index. Skipped a few methods which can also work on datetime index.
