workloadrepo: Use better error codes for errors in the Workload Repository.

[wddevries]

What problem does this PR solve?

Issue Number: ref #58247

Problem Summary:
The workload repository currently returns ErrUnknown(1105) for all errors.

What changed and how does it work?

This pull request:

• adds an error message in createPartition() for failures when generating partition definitions
• adds error handing to generatePartitionDef()
• changes variable handers to return ErrWrongValueForVar when value parsing fails
• changes the workload repository to return ErrNotSupportedYet when enabled without an etcd server
• updates the variables test to validate the changes.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[tiprow]

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[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 15 lines in your changes missing coverage. Please review.

Project coverage is 73.6211%. Comparing base (9d0fb30) to head (0adc651).
Report is 14 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58866        +/-   ##
================================================
+ Coverage   73.1094%   73.6211%   +0.5116%     
================================================
  Files          1676       1677         +1     
  Lines        463866     467082      +3216     
================================================
+ Hits         339130     343871      +4741     
+ Misses       103877     102297      -1580     
- Partials      20859      20914        +55     

Flag	Coverage Δ
integration	42.8036% <0.0000%> (?)
unit	72.2985% <44.4444%> (-0.0199%)	⬇️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.7359% <ø> (-0.0090%)	⬇️

[xhebox]

Use failpoint.Inject functions? Those functions will be enabled on CI, but eliminated when building actual server.