re-implement missing extension error on add_model
#184
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Closes #182. A duplicate PR of #175, and that explanation applies, except that this PR is implemented anticipating that workflows will be released before parsnip. This error will thus "kick in" once the new parsnip is on CRAN, whenever that is, and functionality won't change until then.
PRing as draft, in case it ends up looking like we can get parsnip out with the changes in tidymodels/parsnip#836 before dev workflows is submitted to CRAN.