[API] If User is Admin, show 500 error message on PROD mode too

[6543]

as title + one nit missed in #13108

[codecov-io]

Codecov Report

Merging #13115 into master will increase coverage by 0.12%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master   #13115      +/-   ##
==========================================
+ Coverage   42.25%   42.37%   +0.12%     
==========================================
  Files         675      675              
  Lines       74520    74520              
==========================================
+ Hits        31485    31576      +91     
+ Misses      37922    37812     -110     
- Partials     5113     5132      +19     

Impacted Files	Coverage Δ
models/migrations/migrations.go	2.44% <0.00%> (ø)
modules/context/api.go	75.22% <50.00%> (ø)
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
services/pull/check.go	47.69% <0.00%> (-2.31%)	⬇️
models/error.go	34.39% <0.00%> (ø)
services/pull/pull.go	40.78% <0.00%> (+0.49%)	⬆️
modules/log/event.go	60.84% <0.00%> (+0.94%)	⬆️
services/mailer/mail.go	55.91% <0.00%> (+1.07%)	⬆️
services/gitdiff/gitdiff.go	74.48% <0.00%> (+1.63%)	⬆️
modules/charset/charset.go	70.78% <0.00%> (+2.24%)	⬆️
... and 8 more

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[6543]

🚀

[lunny]

That means Gitea admin should be also system admin.