ERR: Raise on invalid na_action in Series.map
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

`Series.map` treats any `na_action` other than `"ignore"` like `None`, but it should probably raise if someone provides an invalid option:

```python
In [1]: import pandas as pd                                                                                                                                

In [2]: pd.Series([1, 2, 3]).map(lambda x: x, na_action="xxxxx")                                                                                           
Out[2]: 
0    1
1    2
2    3
dtype: int64
```