Add tests for cols_units() and fix edge case + improve error messages for cells_grand_summary()

[olivroy]

Summary

Since it could detect the end of the string in unit, this would fail

library(gt)
d <- data.frame(col1 = 1, col2 = 2)
d |> gt() |> cols_units(everything() ~ "1")
#> Error in if (column_pattern == "" && grepl(units, column_label, fixed = TRUE)) { : 
#>   missing value where TRUE/FALSE needed

due to the fact that the gsub part is TRUE.

Let me know if you want me to buildignore the cols_units() tests.

As always, this occured randomly to me. None of this is related to what I intended to do in the first place, but caught previous mistakes (made by me) and some unfound stuff!

Related GitHub Issues and PRs

Also improve some messages and add some basic checking of input.

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.

[rich-iannone]

LGTM!

[olivroy]

@rich-iannone is it possible to have a gt group and display tables side-by-side? I couldn't find much examples of gt_group()

[rich-iannone]

@olivroy There's nothing built in to make this possible, though it seems like would be a really nice feature down the road.

[rich-iannone]

@olivroy We should do exactly this: #1288