Provide a better error message for non-existent summary or grand summary rows

[olivroy]

Summary

For discussion. I need to think about this a bit more. Maybe could use summary it can find.

A case like the following could be considered too.

My PR seems to address a very narrow problem compared to what's proposed in #1608.

# Create a table from `mtcars` that has all the different components
data <-
  mtcars %>%
  gt(rownames_to_stub = TRUE) %>%
  cols_move_to_start(columns = c("gear", "carb")) %>%
  tab_stubhead(label = "cars") %>%
  cols_hide(columns = "mpg") %>%
  cols_hide(columns = "vs") %>%
  tab_row_group(
    label = "Mercs",
    rows = contains("Merc"),
  ) %>%
  tab_row_group(
    label = "Mazdas",
    rows = contains("Mazda"),
  ) %>%
  tab_options(row_group.default_label = "Others") %>%
  tab_spanner(
    label = "gear_carb_cyl",
    id = "gcc",
    columns = c(gear, carb, cyl)
  ) %>%
  row_group_order(groups = c("Mazdas", "Mercs")) %>%
  cols_merge_range(
    col_begin = "disp",
    col_end = "drat"
  ) %>%
  tab_header(
    title = "Title",
    subtitle = "Subtitle"
  ) %>%
  tab_source_note(source_note = "this is a source note") %>%
  summary_rows(
    groups = c("Mazdas", "Mercs"),
    columns = c(hp, wt, qsec),
    fns = list(
      ~mean(., na.rm = TRUE),
      ~sum(., na.rm = TRUE)
    )
  ) %>%
  grand_summary_rows(
    columns = c(hp, wt),
    fns = list(
      ~mean(., na.rm = TRUE),
      ~sum(., na.rm = TRUE)
    )
  )
# My PR doesn't solve the fact that you can't send all summaries here due to not having summary rows everywhere.
data %>%
    tab_footnote("footnote in summary", cells_summary())
#> Error in `tab_footnote()`:
#> ! Can't add footnote "footnote in summary".
#> Caused by error:
#> ! `everything()` must be used within a *selecting* function.
# need the following:
data %>%
    tab_footnote("footnote in summary", cells_summary(c(1, 3)))

Related GitHub Issues and PRs

• Ref: Support use of non-existent cells_*() within tab_style(locations = ...) #1608

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.