Improve error message when attempting to add a footnote to a hidden column or ignore footnotes added to hidden col

[olivroy]

I encountered this error in a more complex context, but I figured out how to make a reproducible example!

exibble %>% 
  gt() %>% 
  cols_hide(num) %>% 
  tab_footnote("X", locations = cells_column_labels(num)) 
#> Error in `dplyr::mutate()`:
#> ℹ In argument: `fs_id_coalesced = paste(fs_id, collapse = ",")`.
#> ℹ In group 1: `colname = "num"`.
#> Caused by error:
#> ! object 'fs_id' not found

I double-checked that this error occurs in both gt 0.10.1 and gt 0.11, so not due to my refactoring!

[rich-iannone]

For this one, I don't think an error ought to occur. We can add footnotes to visible columns, hide those columns, and gt would know to skip footnotes in those columns. Also, we can reverse the hiding of columns with cols_unhide() so any footnotes placed on hidden data could become visible again (if there wasn't an error to begin with).