Create informative error message if a model engine package is not installed

[markbneal]

Following the example at https://rviews.rstudio.com/2019/06/19/a-gentle-intro-to-tidymodels/, running a model generates uninformative errors if the engine is not installed:

 iris_ranger <- rand_forest(trees = 100, mode = "classification") %>%
  set_engine("ranger") %>%
  fit(Species ~ ., data = iris_training)

If ranger not installed:

# Error in parse(text = text, keep.source = FALSE) : 
#   <text>:5:5: unexpected 'else'
# 4:     }
# 5:     else
#   ^

and for:

iris_rf <-  rand_forest(trees = 100, mode = "classification") %>%
  set_engine("randomForest") %>%
  fit(Species ~ ., data = iris_training)

If randomForest not installed:

# Error in as.data.frame.default(x) : 
#   cannot coerce class ‘c("rand_forest", "model_spec")’ to a data.frame

[juliasilge]

Related to #289

[github-actions]

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