ERR: Raise error in to_excel when saving datetimes with timezones
- [x] closes #27008
- [x] closes #7056
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Raise a consistent error, regardless of the writing engine, when saving datetimes with timezones in Excel since they are unsupported.
cc @WillAyd  Hello @mroeschke! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-06-29 17:08:50 UTC