check recipe argument in check_step_check_args()

[EmilHvitfeldt]

we should add a check in check_step_check_args() to make sure that the recipe argument contains a recipe.

This will also make it so calling step_scale(all_numeric()) by itself (from missing pipe) gives better error

library(recipes)

recipe(mpg ~ ., mtcars)
#> 
#> ── Recipe ──────────────────────────────────────────────────────────────────────
#> 
#> ── Inputs
#> Number of variables by role
#> outcome:    1
#> predictor: 10
  step_scale(all_numeric())
#> Error in `*tmp*`$steps: $ operator is invalid for atomic vectors