Error if `-solution` chunk has an unneeded `exercise.setup` option

[rossellhayes]

Using an exercise.setup option in a -solution results in an error if the chunk is not referenced in the exercise.setup option of another chunk.

---
title: "Tutorial"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)

Exercise 1

x <- 1

Exercise 2

y <- x + 1

Exercise 3

z <- x + y

``` r
rmarkdown::run("setup-chaining.Rmd")
#> processing file: setup-chaining.Rmd
#> Quitting from lines 35-36 (setup-chaining.Rmd)
#> Error: Chunk 'exercise-3-solution' is not being used by any exercise or exercise setup chunk.
#> Please remove chunk 'exercise-3-solution' or reference 'exercise-3-solution' with `exercise.setup = 'exercise-3-solution'`

^{Created on 2023-04-14 with reprex v2.0.2}

This can cause problems for authors who are testing an unfinished exercise that uses setup chaining. They will have to remember not to add an exercise.setup option to exercise-3-solution until after they've written exercise-4.

Would it cause any issues if this threw a warning instead of an error?

[nischalshrestha]

This error actually occurs anytime there is a dangling final setup chunk not being used anywhere else in the tutorial:

---
title: "Tutorial with unused setup chunk"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)

x <- 2

y <- x + 1

will yield:

Error: Chunk 'second_setup' is not being used by any exercise or exercise setup chunk. Please remove chunk 'second_setup' or reference 'second_setup' with exercise.setup = 'second_setup'

This error was initially put in place just as a safeguard that users might accidentally forget to use a setup chunk, but authors might want a warning instead to nudge them instead of totally halting rendering of document to check other things.

But, we'll need to look at [these lines](https://github.com/rstudio/learnr/blob/1ffad2a752b21a782da54bd243221e325f489e0c/R/knitr-hooks.R#L127-L133) and that general function for chained setup chunks to verify if there are other reasons to error. 🤔