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

[NobbZ]

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:

$ 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:

{"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.