Throw an informative error if an exercise chunk is NULL

[cpsievert]

Closes #172
Closes #410

Testing notes

Install remotes::install_github("rstudio/learnr") and run the following tutorial (rmarkdown::run("test.Rmd")). After these changes, you should now get the following error (i.e., the test passes if you see this error):

Error: The exercise chunk 'foo' doesn't have anything inside of it. Try adding empty line(s) inside the code chunk.

---
title: "Issue #410"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)
library(gradethis)
tutorial_options(exercise.checker = grade_learnr)
knitr::opts_chunk$set(echo = FALSE)
```

## Minimal example

This chunk isn't treated as an exercise!

```{r foo, exercise = TRUE}
```

```{r foo-solution}
sum(1)
```

```{r foo-check}
grade_code()
```

[schloerke]

Tested on #172. ✅

❯❯ rmarkdown::run("learnr_issue_172.Rmd")
Loading required namespace: shiny


processing file: learnr_issue_172.Rmd
  |........                                                                                                      |   7%
  ordinary text without R code

  |................                                                                                              |  14%
label: setup (with options) 
List of 1
 $ include: logi FALSE

  |........................                                                                                      |  21%
  ordinary text without R code

  |...............................                                                                               |  29%
label: dataframe-exercise (with options) 
List of 1
 $ exercise: logi TRUE

  |.......................................                                                                       |  36%
  ordinary text without R code

  |...............................................                                                               |  43%
label: dataframe-exercise-hint-1
  |.......................................................                                                       |  50%
  ordinary text without R code

  |...............................................................                                               |  57%
label: dataframe2-exercise (with options) 
List of 1
 $ exercise: logi TRUE

Quitting from lines 31-31 (learnr_issue_172.Rmd) 
Error: The exercise chunk 'dataframe2-exercise' doesn't have anything inside of it. Try adding empty line(s) inside the code chunk.

[schloerke]

Please add a news item, as it closes #172