Add explanatory error message for VariableNotOwned errors

[odow]

Closes #3517

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10% 🎉

Comparison is base (1ee5fae) 98.14% compared to head (97145bd) 98.25%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3520      +/-   ##
==========================================
+ Coverage   98.14%   98.25%   +0.10%     
==========================================
  Files          37       37              
  Lines        5548     5550       +2     
==========================================
+ Hits         5445     5453       +8     
+ Misses        103       97       -6     

Files Changed	Coverage Δ
src/variables.jl	99.02% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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