expand & improve error message for grouping by unsupported types

[MichaelChirico]

Closes #4308

[codecov]

Codecov Report

Merging #4309 (848ddeb) into master (5421e7a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 848ddeb differs from pull request most recent head f50677f. Consider uploading reports for the commit f50677f to get more accurate results

@@           Coverage Diff           @@
##           master    #4309   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files          75       75           
  Lines       14807    14808    +1     
=======================================
+ Hits        14729    14730    +1     
  Misses         78       78           

Impacted Files	Coverage Δ
R/data.table.R	99.94% <100.00%> (+<0.01%)	⬆️

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