make error context more informative for ranger test

[simonpcouch]

parsnip/tests/testthat/_snaps/rand_forest_ranger.md

Lines 19 to 25 in 4562458

	Code
	parsnip:::predict_classprob.model_fit(no_prob_model, new_data = lending_club[1:
	6, num_pred])
	Condition
	Error in `object$spec$method$pred$prob$pre()`:
	! `ranger` model does not appear to use class probabilities.
	i Was the model fit with `probability = TRUE`?

Noticed this in the transition to snapshot testing in #1190.

[github-actions]

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