simplify + provide examples [defense]

[lindsayplatt]

• Common gotchas: it would be nice to have examples for the suggested functions to use instead (e.g., isTRUE() and assert_that()) to show how they would be implemented in a function. I got distracted because it wasn't immediately clear to me what those functions were doing or how I would implement them.
• Fail conspicuously: like the comment above, it would be super useful to walk through examples of how to implement conditions - as it's a bit hard to conceptualize without seeing them within a function.
• I really like the examples in Fail Informatively - but it took me a while to wrap my head around what they were doing because it used so many unfamiliar commands (tryCatch, switch, sprintf), some of which are not related to defensive programming. Maybe simplify so no one gets caught up trying to understand the function itself? Probably fine in the online tutorial since people can take some time, but in a live training might struggle with it.

Comes from #206