DOC: show failing string on numeric parse
- [ ] related to #13746
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry not needed?

``` python
# before
In [7]: pd.to_numeric(['a'])
ValueError: Unable to parse string

# after
In [1]: pd.to_numeric(['a'])
ValueError: Unable to parse string "a" at position 0
```
