feat(gen): improve discriminator inference error reporting

[tdakkota]

For #1185

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 73.84%. Comparing base (8bf8791) to head (6961361).
Report is 1 commits behind head on main.

Files	Patch %	Lines
gen/schema_gen_sum.go	93.33%	1 Missing and 1 partial ⚠️
gen/errors.go	88.88%	1 Missing ⚠️
location/error.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1188      +/-   ##
==========================================
+ Coverage   73.78%   73.84%   +0.05%     
==========================================
  Files         188      188              
  Lines       12661    12692      +31     
==========================================
+ Hits         9342     9372      +30     
- Misses       2785     2786       +1     
  Partials      534      534              

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