Parse error receive "parse_error" class

[gadenbuie]

Adds a "parse_error" class to the error thrown when trying to parse the user's submitted code. This makes it possible for the error checker — which now gets first dibs on the parse error before the default parse error feedback is returned — to differentiate between parse errors and other errors.

# in "base" learnr error checking code
inherits(last_value, "parse_error")

# in gradethis error checking code
inherits(.result, "parse_error")

[rossellhayes]

Looks good to me!