Improve error output

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
