Add better error message when index is same as model

[odow]

Closes #858

[codecov]

Codecov Report

Merging #2486 (ef9b8b7) into master (9daea7d) will increase coverage by 0.02%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master    #2486      +/-   ##
==========================================
+ Coverage   92.38%   92.40%   +0.02%     
==========================================
  Files          43       43              
  Lines        4663     4676      +13     
==========================================
+ Hits         4308     4321      +13     
  Misses        355      355              

Impacted Files	Coverage Δ
src/macros.jl	92.50% <80.95%> (+0.19%)	⬆️
src/constraints.jl	93.46% <0.00%> (+0.04%)	⬆️

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