Descriptive error message for anonymous `gin get`

The gin-cli only permits to `get` public repositories when being logged in. The error message when trying to `get` a repository anonymously is
```sh
Downloading repository Repository download failed. Internal git command returned: Cloning into ...
git@gin.g-node.org: Permission denied (publickey,keyboard-interactive).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

[error] 1 operation failed
```

I think it would be helpful to not only state this as a `permission denied` error, but also provide the hint that logging in using `gin login` might resolve the issue. This issue is related to #52