The function already returns early if the specified 'netrc' configuration points to a non existing file, but currently returns an error if the OS reports ENOTDIR:

This will happen if the $HOME directory of the specified user points to a file instead a directory - something eg. 'void linux' does for user 'nobody':

```
$ grep nobody /etc/passwd
nobody:x:99:99:Unprivileged User:/dev/null:/bin/false
```

go-getter then attempts to open `/dev/null/.netrc` which fails with ENOTDIR - something that should just be threated the same way as a non existing file (in this case)

Make addAuthFromNetrc ignore ENOTDIR errors