Remove alternate custom error message mechanism

[justin-tay]

This removes the alternate custom error message mechanism. This mechanism isn't really documented and cannot be disabled.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.02%. Comparing base (48ca3c2) to head (327665b).
Report is 46 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1083      +/-   ##
============================================
- Coverage     78.90%   78.02%   -0.88%     
- Complexity     1965     2059      +94     
============================================
  Files           172      201      +29     
  Lines          6352     6868     +516     
  Branches       1255     1254       -1     
============================================
+ Hits           5012     5359     +347     
- Misses          867     1036     +169     
  Partials        473      473              

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