Provides instructions on parse error caused by code scaffolding in learnr tutorial

[gadenbuie]

Fixes #134

Currently, the code scaffolding commonly used in exercises, e.g. ____(mtcars, ____), causes a parse error, resulting in an error message like the one below:

This PR adds logic to the default user code error handler in grade_learnr() to detect three or more _ in the user code and produce an informative error message with instructions.

Because the logic adds a bit of complexity to the error handler, I moved the default handler into a new function: grade_learnr_parse_error().

[gadenbuie]

Fixes #134

[schloerke]

Thank you!

[gadenbuie]

@schloerke @garrettgman this is ready for re-review.

In short, this PR adds grade_parse_error() an internal function that demonstrates how parse errors can be handled via the exercise.parse.error option. The option accepts a function with a single argument.

The only argument to the parse error function is the gradethis check environment, typically called check_obj, which provides access to everything gradethis knows about the exercise. Most relevant are .user_code and .error which provide a character string of the user-entered code and the error condition thrown from the parse failure. (These are elements in check_obj and are accessed via check_obj$.user_code.)

Finally, the parse error function should return a fail() (or a `pass() if you want) with an appropriate message.

[schloerke]

LGTM given small changes!