FEATURE REQUEST for grade_this_code()

[garrettgman]

I'd like to screen for particular failure modes in grade_this_code() to provide custom messages.

For example, in the example below, I'd like to provide a custom message instead of code_feedback() when the user submits censored[2, 3, 6].

```{r censored}
censored <- c(TRUE, NA, NA, FALSE, FALSE, NA, TRUE)
censored
```

```{r ref.label = "censored"}

```

```{r ph, exercise = TRUE, exercise.setup = "censored"}
censored
```

```{r ph-solution}
censored[c(2, 3, 6)]
```

```{r ph-check}
grade_this_code(
   fail_if_code_is(
    "censored[2, 3, 6]", 
    message = "Did you forget `c()`?", 
    ignore_white_spaces = TRUE
  )
)
```

This may be quickly withdrawn because I bet we can do it with a specific incantation in grade_this(), but might it be more helpful in grade_this_code()?