better error message for missing j in cube

[MichaelChirico]

Ran into this today -- before this fix, the missing j was being passed as jj to groupingsets and we'd get an error from all.vars referencing a missing jj. Better to reference j since that's part of args(cube.data.table).

[codecov]

Codecov Report

Merging #4282 (ad7b1f2) into master (54082e4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head ad7b1f2 differs from pull request most recent head 1e9c8ec. Consider uploading reports for the commit 1e9c8ec to get more accurate results

@@           Coverage Diff           @@
##           master    #4282   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          73       73           
  Lines       14430    14432    +2     
=======================================
+ Hits        14347    14349    +2     
  Misses         83       83           

Impacted Files	Coverage Δ
R/groupingsets.R	100.00% <100.00%> (ø)

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