[mdatagen] better errors when can't get package name

[valner]

Description

Make errors more detailed if mdatagen can't get go package name

errors before

Error: failed loading metadata.yaml: exit status 1

after

Error: failed loading metadata.yaml: failed to run go list -f {{.ImportPath}} command: exit status 1: main module (...) does not contain package ...

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.55%. Comparing base (6aad9d1) to head (fd4d73e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/loader.go	80.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (80.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12682   +/-   ##
=======================================
  Coverage   91.55%   91.55%           
=======================================
  Files         480      480           
  Lines       26391    26394    +3     
=======================================
+ Hits        24162    24165    +3     
  Misses       1766     1766           
  Partials      463      463           

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[ms-jcorley]

LGTM