Error message for rate-limiting

When github returns a 403 when you try and download a tarball, it returns a message saying something like:

```shell
Unable to find module 'foo/bar' on https://github.com
```

A better message would to say "The reason this failed is because of a 403 response, you need to add an API key etc"

This is a very basic hacky way of implementing that, would like to get your thoughts?