When epi_recipe() outputs a regular recipe, errors can be confusing

[brookslogan]

If epi_recipe() is provided data that's not an epi_df, it will output a regular recipe rather than an epi_recipe, causing some step functions to output somewhat confusing messages.

Artificial example:

library(epipredict)
#> Loading required package: epiprocess
#> 
#> Attaching package: 'epiprocess'
#> The following object is masked from 'package:stats':
#> 
#>     filter
#> Loading required package: parsnip
library(tibble)
epi_recipe(as_tibble(case_death_rate_subset)) %>%
  step_epi_ahead(death_rate, ahead = 7) %>%
  step_epi_lag(death_rate, lag = c(0,7,14))
#> Error in `step_epi_ahead()`:
#> ! This recipe step can only operate on an `epi_recipe`.

^{Created on 2023-04-26 by the reprex package (v2.0.1)}

More realistically, @dshemetov and I ran into this when feeding in a covidcast_signal before it had been converted to epi_df format.

A couple quick ideas on potential resolutions:

• Add S3 methods to the steps for regular recipes, just to give better error messages.
• Always output an epi_recipe from epi_recipe(), but complain in step functions if the appropriate roles have not been set. This might require fixing some hard-coded references to geo_value and/or time_value columns (rather than selecting based on roles) in some steps.

[dajmcdon]

I would lean toward the second option. This potentially interacts with #175 .