better error when formula variables missing from dataset

[palday]

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.
• I've bumped the version appropriately

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (563d465) 95.83% compared to head (03772bc) 95.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #707   +/-   ##
=======================================
  Coverage   95.83%   95.83%           
=======================================
  Files          35       35           
  Lines        3263     3266    +3     
=======================================
+ Hits         3127     3130    +3     
  Misses        136      136           

Flag	Coverage Δ
current	95.83% <100.00%> (+<0.01%)	⬆️
minimum	95.73% <100.00%> (+<0.01%)	⬆️
nightly	95.83% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
src/linearmixedmodel.jl	95.00% <100.00%> (+0.02%)	⬆️

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