Export InvalidIndexError
- [x] closes #xxxx (there is no such an issue)
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@horta  there is a `pandas.errors` submodule where this could be put instead of at the top-level namespace I have now defined `InvalidIndexError` at `pandas.errors` module. Documented it (suggestions?). Most of the diff concerns importing `InvalidIndexError` from `pandas.errors`. Could you include this error in the API reference? https://pandas.pydata.org/docs/reference/general_utility_functions.html#exceptions-and-warnings @horta looks fine, can you rebase and update to the comments above, ping on green. > Could you include this error in the API reference? https://pandas.pydata.org/docs/reference/general_utility_functions.html#exceptions-and-warnings

Done. lgtm ping on green.