fail_if() gives a cryptic message if cond errors

[rossellhayes]

grade_this({
  fail_if(!all.equal(.result, .solution), "Oops!")
})(mock_this_exercise("1", "2"))
#> #> grade_this({
#> #>     fail_if(!all.equal(.result, .solution), "Oops!")
#> #> })(mock_this_exercise("1", "2"))
#> Error in rlang::is_formula(x): invalid argument type
#> <gradethis_graded: [Neutral]
#>   A problem occurred with the grading code for this exercise.
#> >

^{Created on 2023-04-04 with reprex v2.0.2}