fix: local BE - don't leave broken files when writing fails

[aawsome]

see rustic-rs/rustic#1731

[codecov]

Codecov Report

❌ Patch coverage is 45.28302% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.3%. Comparing base (98d430e) to head (5cf374f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/backend/src/local.rs	47.3%	20 Missing ⚠️
crates/core/src/backend/cache.rs	40.0%	9 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/backend/cache.rs	59.6% <40.0%> (-0.9%)	⬇️
crates/backend/src/local.rs	41.9% <47.3%> (-0.5%)	⬇️

... and 16 files with indirect coverage changes

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