cksum, hashsum: Manually error standalone --warn's and comment why #10115

Performance Regression: -4.25%

❌ 1 regressed benchmark
✅ 141 untouched benchmarks
⏩ 37 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

	Benchmark	`BASE`	`HEAD`	Efficiency
❌	`cksum_blake3`	218.9 µs	228.6 µs	-4.25%

_{Comparing oech3:warn-bad-workaround (3c821b2) with main (30239e6)}

37 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. ↩