refactor: refine error downcast related API

[MrCroxx]

Signed-off-by: MrCroxx [email protected]## What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed. :D

Tried #1059 with OpenDAL, do some tiny refactorings.

Checklist

• I have written the necessary rustdoc comments
• I have added the necessary unit tests and integration tests
• I have passed cargo x (or cargo x --fast instead if the old tests are not modified) in my local environment.

Related issues or PRs (optional)

#1060
#1059
apache/opendal#6366

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer/src/hybrid/error.rs	42.85%	4 Missing ⚠️

Files with missing lines	Coverage Δ
foyer/src/hybrid/error.rs	58.33% <42.85%> (-4.53%)	⬇️

... and 1 file with indirect coverage changes

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