fix(nydusd): fix parsing of failover-policy argument

[BraveY]

Use inspect_err instead of inspect to to correctly handle and log errors when parsing the failover-policy argument.

Relevant Issue (if applicable)

If there are Issues related to this PullRequest, please list it.
Fix the bug that nydusd reports error:Invalid failover policy in CI.

[2025-02-21 03:17:22.252830 +00:00] ERROR [src/bin/nydusd/main.rs:508] Invalid failover policy
[2025-02-21 03:17:22.253245 +00:00] ERROR [src/bin/nydusd/main.rs:508] Invalid failover policy
[2025-02-21 03:17:22.253671 +00:00] ERROR [src/bin/nydusd/main.rs:508] Invalid failover policy
[2025-02-21 03:18:42.554628 +00:00] ERROR [src/bin/nydusd/main.rs:508] Invalid failover policy

Details

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Types of changes

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• 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

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.41%. Comparing base (dd9ba54) to head (288a383).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/bin/nydusd/main.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1669      +/-   ##
==========================================
- Coverage   60.42%   60.41%   -0.01%     
==========================================
  Files         146      146              
  Lines       48943    48943              
  Branches    46380    46380              
==========================================
- Hits        29573    29570       -3     
- Misses      17906    17908       +2     
- Partials     1464     1465       +1     

Files with missing lines	Coverage Δ
src/bin/nydusd/main.rs	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes