Add validationIssue method

[landonreed]

@robertgregg3, here are some changes for #668 that I think will clean things up a bit. I've just added a method to encapsulate the EditorValidationIssue return value, which gets reused quite a bit in this method.

[codecov-commenter]

Codecov Report

Merging #690 (5d17fbb) into gtfs-spec-changes (7717c28) will increase coverage by 0.01%.
The diff coverage is 30.76%.

@@                  Coverage Diff                  @@
##           gtfs-spec-changes     #690      +/-   ##
=====================================================
+ Coverage              15.67%   15.68%   +0.01%     
=====================================================
  Files                    329      329              
  Lines                  16820    16822       +2     
  Branches                5069     5070       +1     
=====================================================
+ Hits                    2636     2638       +2     
  Misses                 12133    12133              
  Partials                2051     2051              

Flag	Coverage Δ
unit_tests	15.68% <30.76%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
lib/editor/util/validation.js	44.44% <30.76%> (+1.14%)	⬆️

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