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`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13624?src=pr) is **84.33%**

> Merging [#13624](https://codecov.io/gh/pydata/pandas/pull/13624?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13624   diff @@
==========================================
  Files           138        138          
  Lines         51100      51104     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43097      43101     +4   
  Misses         8003       8003          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [65849d3...856df95](https://codecov.io/gh/pydata/pandas/compare/65849d3c9feea395d6e6a124f7a3b11ecdb943cb...856df9517322d6547a3a9e664daac5cdb89554d1?src=pr)
