Checking for exercise errors

[cpsievert]

Closes #356. Adds a new option exercise.error.checker for customizing the feedback provided when an exercise submission results in an evaluation error (for control over feedback when parsing user code results in error, see rstudio/gradethis#123). This option must be a function that anticipates the same arguments as exercise.checker and return a feedback object. A recommend default for this option is the new gradethis::grade_learnr_error function.

Testing notes

Install remotes::install_github("rstudio/learnr") and remotes::install_github("rstudio-education/gradethis") and run the following tutorial (rmarkdown::run("test.Rmd")). Verify that entering stop("boom") and clicking "Submit Exercise" returns in positive (green) feedback and that entering anything else results in sensible/informative negative (red) feedback.

---
title: "Checking errors"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)	
library(gradethis)
tutorial_options(
  exercise.checker = grade_learnr, 
  exercise.error.checker = grade_learnr_error
)
```

* Throw an error message of `"boom"`:

```{r grade_error, exercise = TRUE}

```

```{r grade_error-solution}
stop("boom")
```

```{r grade_error-code-check}
grade_code("Nice error!")
```

[schloerke]

LGTM with suggestions