add details in APIError.Error

[randmonkey]

Because sync.Solve wraps the error returned from CRUD actions by fmt.Errorf(), the details which are not included in APIError.Error() method is lost in wrapping errors. So Kong/kubernetes-ingress-controller#5274 requires details to be included in Error() method.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (19b8bfa) 53.18% compared to head (bc8ff75) 53.19%.

Files	Patch %	Lines
kong/error.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #400   +/-   ##
=======================================
  Coverage   53.18%   53.19%           
=======================================
  Files          69       69           
  Lines        5259     5262    +3     
=======================================
+ Hits         2797     2799    +2     
- Misses       1876     1877    +1     
  Partials      586      586           

Flag	Coverage Δ
2.1	33.37% <75.00%> (+0.01%)	⬆️
2.2	44.71% <75.00%> (+0.01%)	⬆️
2.3	45.41% <75.00%> (+0.01%)	⬆️
2.4	45.49% <75.00%> (+0.01%)	⬆️
2.5	45.49% <75.00%> (+0.01%)	⬆️
2.6	45.49% <75.00%> (+0.01%)	⬆️
2.7	46.86% <75.00%> (-0.11%)	⬇️
2.8	46.97% <75.00%> (+0.01%)	⬆️
3.0	50.66% <75.00%> (+<0.01%)	⬆️
3.1	52.10% <75.00%> (+<0.01%)	⬆️
3.2	52.16% <75.00%> (+<0.01%)	⬆️
3.3	52.16% <75.00%> (+<0.01%)	⬆️
3.4	52.48% <75.00%> (+<0.01%)	⬆️
community	38.54% <75.00%> (+0.01%)	⬆️
enterprise	51.88% <75.00%> (+<0.01%)	⬆️
enterprise-nightly	50.85% <75.00%> (+<0.01%)	⬆️
integration	53.19% <75.00%> (+<0.01%)	⬆️
nightly	37.83% <75.00%> (+0.01%)	⬆️

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[randmonkey]

Close this because we can extract APIError from errors returned in sync.Solve in KIC.