refactor: export BoundTypeError with Fset and Pos fields

[aofei]

Updates goplus/builder#1378

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.94%. Comparing base (5afc508) to head (05916ad).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
template.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #472   +/-   ##
=======================================
  Coverage   95.94%   95.94%           
=======================================
  Files          19       19           
  Lines        7202     7206    +4     
=======================================
+ Hits         6910     6914    +4     
  Misses        228      228           
  Partials       64       64           

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