transition warning to error when predicting from a failed model fit

[simonpcouch]

There's a good bit of this throughout the source:

parsnip/R/predict_raw.R

Lines 18 to 21 in 8af5ddf

	if (inherits(object$fit, "try-error")) {
	cli::cli_warn("Model fit failed; cannot make predictions.")
	return(NULL)
	}

A couple thoughts:

1. This warning likely ought to be an error.
2. We can set parent = object$fit so that the model fit error is surfaced as part of the message.
3. This pattern should live inside of a helper.