Improve error for non-constant Interval constraints

[odow]

Closes #2599

[codecov]

Codecov Report

Merging #2712 (a9eef23) into master (a9890a4) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2712      +/-   ##
==========================================
+ Coverage   93.54%   93.61%   +0.07%     
==========================================
  Files          44       44              
  Lines        5456     5454       -2     
==========================================
+ Hits         5104     5106       +2     
+ Misses        352      348       -4     

Impacted Files	Coverage Δ
src/macros.jl	93.84% <100.00%> (+0.35%)	⬆️
src/nlp.jl	93.75% <100.00%> (+0.27%)	⬆️

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