Cannot read property 'toString' of null when `git` is missing

When the git executable is missing, `gitDescribeSync` fails with the rather cryptic:

```
TypeError: Cannot read property 'toString' of null
    at handleProcessResult (...../node_modules/git-describe/lib/git-describe.js:70:20)
```

(you can reproduce this easily by changing the name of the executable on line 40)

It would be excellent if we could get a clearer error thrown in that case.