validate.rs: extract actual error message from OS

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will improve performance by 3.95%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 3 improved benchmarks
✅ 354 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	du_summarize_balanced_tree[(5, 4, 10)]	16.8 ms	16 ms	+4.59%
⚡	Simulation	cksum_sysv	68.4 ms	65.7 ms	+4.16%
⚡	Simulation	cksum_crc32b	39.7 ms	38.5 ms	+3.11%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing oech3:cksum-err (4fee4df) with main (0aeda8f)}

Footnotes

1. 50 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.

[sylvestre]

is it really worse the added complexity?