feat: optimize error message of preheating

[gaius-qi]

Description

This pull request focuses on improving error handling in the manager package by providing more detailed error messages. The most important changes include updating error messages in the Error middleware and enhancing error context in the findSchedulers and findCandidateSchedulers service methods.

Error handling improvements:

• manager/middlewares/error.go: Updated the error message in the GORM error handler to use err.Error() instead of a generic HTTP status text.

• manager/service/job.go: Enhanced error context in the findSchedulers method by including the scheduler cluster ID in the error message.

• manager/service/job.go: Enhanced error context in the findCandidateSchedulers method by including the scheduler cluster ID in the error message.

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[fcgxz2003]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 35.58%. Comparing base (19b38a1) to head (7d8ea38).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
manager/service/job.go	0.00%	4 Missing ⚠️
manager/middlewares/error.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #3622       +/-   ##
=========================================
+ Coverage      0   35.58%   +35.58%     
=========================================
  Files         0      335      +335     
  Lines         0    37389    +37389     
=========================================
+ Hits          0    13305    +13305     
- Misses        0    23241    +23241     
- Partials      0      843      +843     

Flag	Coverage Δ
unittests	35.58% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
manager/middlewares/error.go	0.00% <0.00%> (ø)
manager/service/job.go	0.00% <0.00%> (ø)

... and 333 files with indirect coverage changes

[chlins]

lgtm