pipes.rs: replace bool by Result<(),()>

[oech3]

Fix true to check that fallback is needed.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/csplit/csplit-heap is now passing!
Congrats! The gnu test tests/cut/bounded-memory is now passing!

[oech3]

Actually, I don't like this diff

[oech3]

#12463 should be merged before this.

[codspeed-hq]

Merging this PR will degrade performance by 3.34%

❌ 1 regressed benchmark
✅ 318 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	sort_long_line[10000]	424 µs	438.7 µs	-3.34%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing oech3:splice-bool (6fdc839) with main (913c6c2)}

Footnotes

1. 46 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. ↩