GH14422: BUG: to_numeric doesn't work uint64 numbers
-closes #14422
-passes `black pandas`
-added test test_downcast_uint64_exception()
Hello @pv8473h12! 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 2019-11-02 18:41:52 UTC thanks @pv8473h12 for the PR! - mind addressing the pep8 issues? thanks @pv8473h12 