scheduler: enhance reservation error message

[saintube]

Ⅰ. Describe what this PR does

• Enhance reservation error messages for printing unschedulable reasons by reservation-level.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[koordinator-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

Attention: Patch coverage is 87.80488% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 68.60%. Comparing base (7e944d5) to head (8600a51).

Files	Patch %	Lines
pkg/scheduler/plugins/reservation/plugin.go	81.57%	7 Missing ⚠️
pkg/scheduler/plugins/reservation/transformer.go	91.17%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2058      +/-   ##
==========================================
+ Coverage   68.55%   68.60%   +0.05%     
==========================================
  Files         430      430              
  Lines       39387    39445      +58     
==========================================
+ Hits        27000    27061      +61     
+ Misses      10047    10045       -2     
+ Partials     2340     2339       -1     

Flag	Coverage Δ
unittests	68.60% <87.80%> (+0.05%)	⬆️

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[saintube]

Merged by #2063.