vtorc: cleanup init db handling

[timvaillancourt]

Description

This PR does some cleanup to the way errors are handled in vtorc at DB init-time, some details:

• Funcs with unused error return now check return
• log.Fatal(...)s move to the top level caller

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 12 lines in your changes missing coverage. Please review.

Project coverage is 67.31%. Comparing base (a9bab82) to head (0ba72f2).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/db/db.go	42.85%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17198      +/-   ##
==========================================
- Coverage   67.42%   67.31%   -0.11%     
==========================================
  Files        1569     1569              
  Lines      252120   252549     +429     
==========================================
+ Hits       169997   170015      +18     
- Misses      82123    82534     +411     

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