Draft implementation
Raises a ValueError when the input to to_datetime contains some numbers
but the unit isn't supplied.

- [X] closes #15836
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @samuelsinayoko! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 27, 2019 at 20:35 Hours UTC First part of any PR should be tests so please make sure to add them. Also looks like this introduced a slew of errors so be sure to run the suite locally before pushing to GH Closing as stale - ping if you'd like to pick this back up and have tests