More informative messages when fail_if(cond = ) is invalid

[rossellhayes]

pass_if() and fail_if() now produce more informative error messages if their cond argument is invalid.

grade_this({
  fail_if(!all.equal(.result, .solution), "Oops!")
})(mock_this_exercise("1", "2"))
#> Warning in assert_gradethis_condition_does_not_error(cond, from): The `cond` argument to `fail_if()` produced an error:
#>   Error in !all.equal(.result, .solution) : invalid argument type
#> <gradethis_graded: [Neutral]
#>   A problem occurred with the grading code for this exercise.
#> >

grade_this({
  pass_if(all.equal(.result, .solution), "Nice!")
})(mock_this_exercise("1", "2"))
#> Warning in assert_gradethis_condition_is_logical(cond, from): The `cond`
#> argument to `pass_if()` must be coercible to logical, not an object of class
#> <character>.
#> <gradethis_graded: [Neutral]
#>   A problem occurred with the grading code for this exercise.
#> >

grade_this({
  pass_if(NA, "Nice!")
})(mock_this_exercise("1", "2"))
#> Warning in assert_gradethis_condition_is_not_na(cond, from): The `cond`
#> argument to `pass_if()` must not be `NA`.
#> <gradethis_graded: [Neutral]
#>   A problem occurred with the grading code for this exercise.
#> >

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

Closes #340.