Give all NotImplementedErrors a description
Currently some NotImplementedErrors don't have a description (most do!) .

```
raise NotImplemented
# should be
raise NotImplemented("short description of thing which isn't implemented")
```

I think this would be pretty useful, and makes it less confusing for users, and at least they would be pointed in the right direction to request said feature (and know which bit of specifically is NotImplemented)...

You can find them all with grep (probably more usefully the same regex in your text editor):

```
grep "NotImplementedError$" pandas/**/*.py -r -n
```
