BUG: Invalid Timedelta op may raise ValueError
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Timedelta` op may raise `ValueError` for invalid inputs because of internal conversion. 

```
pd.Timedelta('1 days') + 'a'
# ValueError: unit abbreviation w/o a number
```

Now it raises `TypeError`.
