Improve error output

[beauraines]

If a user enters a command in error, e.g. continute rather than continue, the toggl command silently exits. This has resulted in me not starting a time entry as expected. As a toggl user, I want to be alerted if I entery a command in error, so I know that I should fix it.

Check out the [.fail()](https://github.com/yargs/yargs/blob/main/docs/api.md#failfn--boolean) API method