check plausibility of model spec before fitting preprocessor

[simonpcouch]

In fit.workflow(), we can know whether a model fit will fail using parsnip::spec_is_possible(). Calling this before fitting the preprocessor would alleviate the pain of fitting a computationally intensive preprocessor, only for the model fit to then fail.

[EmilHvitfeldt]

here we are forced to use a more unsightly object$fit$actions$model$spec because we don't have a way to detect whether there is a model spec.

we tried to use extract_spec_parsnip() but it gave a less informative error than what we had before

https://github.com/tidymodels/workflows/pull/281/files#diff-89ce8a92518a7ad9368bcb1ad800fecaaf951cda5cb47da4ac6394a50599e453R7