need a better error message for some parsnip functions

[topepo]

> bag_tree %>% set_mode("classification")
Error in `check_spec_mode_engine_val()` at parsnip/R/arguments.R:83:2:
! 'classification' is not a known mode for model `function()`.

We should be able to say "you might need to load packages x, y, z for this engine" so some other information

[qiushiyan]

This happens when mode = "unknown", we have the error message setup in https://github.com/tidymodels/parsnip/blob/main/R/misc.R#L86-L95

Actually this is a separate problem that is fixed after set_mode and set_engine become generic in #769, now the error comes from not finding the method for class "function".

It could be more helpful if we add a default method and if the object is a function say something like "did you mean bag_trees()" with parenthesis in the error. However with magrittr pipes I am not sure how we can extract the name of the object, rlang::ensym() or subtitute() will return ..

[github-actions]

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