Use `cli::cli_abort()` everywhere.

[olivroy]

I also refactored some tests.

I found a test that was failing silently. I will attempt to find the root in a subsequent PR.

I found that some defunct functions were still used in certain places as well.

[olivroy]

@rich-iannone I see there are many issues open. If you want, I could try cleaning it up a little to cleanup a little to make further development a little easier?

[rich-iannone]

@olivroy that would be great, thank you! Also, before we merge this PR, would you be okay with adding yourself as an author to the package?

[olivroy]

Thank you! :) Done!