stack build should check whether git is available

I was running `stack build` on a (virtual) machine, with some github locations in stack.yaml. I got errors like

```
 .stack-work/downloaded/Lcy0nQgWISfb/: getDirectoryContents: does not exist (No such file or directory)
```

The reason is that my machine did not have `git` installed. 

Looks like `stack` called `git`, but did not check the exit (error) code.
