fix: fix panic on scanner overflow

[MrCroxx]

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

• Fix panic on scanner overflow.
• Rename CachedDeviceReader as CachedRegionReader.

Checklist

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

Related issues or PRs (optional)

fix #676

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
foyer-storage/src/error.rs	0.00% <ø> (ø)
foyer-storage/src/large/scanner.rs	75.72% <100.00%> (+7.06%)	⬆️
foyer-storage/src/region.rs	90.60% <100.00%> (+0.15%)	⬆️

... and 2 files with indirect coverage changes