Default feedback for "fill in the blanks" blanks left in exercise

[mine-cetinkaya-rundel]

Many tutorial authors use exercises where they have a "fill in the blanks" type scaffolding for learners to start with, e.g.

ggplot(data = ___, aes(x = mpg, y = ___) +
  ___

Usually, these are 3 underscores, but different authors might use different placeholders for the blanks.

It would be nice if gradethis had a default message if learner left any ___ in their submission, like "Please replace all blanks with valid code." It's a bit of a pain to write code checking for existence of blanks and makes the -check chunk repetitive throughout the document, and this would help. I think tutorial authors would be fine if the package required a certain type of representation for blanks (e.g., 3 underscores) but it might also be nice if the authors could set a gradethis option within a tutorial to indicate the text string they're using to indicate blanks.