Fix config file not found handling

[GuptaManan100]

Description

As reported in #17577, there was a bug in the code which wasn't catching file not found errors if the config file is explicitly provided.
This PR fixes this issue and adds tests to ensure that everything works as intended.

Related Issue(s)

• Fixes Bug Report: ConfigFileNotFoundHandling doesn't get triggered if the file doesn't exist #17577

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]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.71%. Comparing base (10ff5e3) to head (9092420).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
go/viperutil/config.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17578   +/-   ##
=======================================
  Coverage   67.70%   67.71%           
=======================================
  Files        1585     1585           
  Lines      254833   254903   +70     
=======================================
+ Hits       172547   172609   +62     
- Misses      82286    82294    +8     

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