When inexisting `--track`/`--exercise` options are provided, CLI does not error

# Current behaviour

When one provides an inexisting combination of `track` and `exercise` switches on the terminal, then the CLI will report successful download to `$(exercism workspace)/users`:

```plain
$ exercism download --track=erlang --exercise=hello

Downloaded to
/home/nmelzer/exercism/users
```

After the download there will be a `.exercism` folder with a `metadata.json` in it. It has the following contents:

```json
{"track":"","exercise":"","id":"","url":"","handle":"","is_requester":false,"auto_approve":false}
```

# Expected behaviour

The CLI should tell us with an error message that the track/exercise does not exist.