Unit testing for LoquiAPI

[cansavvy]

Related to #3 but a little different.

We should try to think of all the scenarios that loquiAPI could fail on and make sure we have:

1. A testthat test to look out for that
2. an error message to handle it that informs the user on how to fix it -- or if it can't be fixed by the user that they should post an issue here.

Anything that can break loquiAPI should have some handling around it. Some examples of handling that need to be added (but this isn't a comprehensive list)

• Slides not publicly shared.
• That mysterious ffmpeg error.
• Is a slide doesn't have text (or I think maybe this is fixed now?).
• If arguments provided are not specified correctly.

[cansavvy]

I'm happy to help build tests, but we should pair on this @howardbaek So you can tell me what are the most common ways loquiAPI breaks.