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
```

I think its straightforward to also provide a row/line number here, e.g. `i` which we know already
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13773?src=pr) is 85.23% (diff: 100%)

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 @jorisvandenbossche -  I think this one is good now.
 Thanks!
