dd: name the failing operation and count the read that failed to write

[igorgbr]

Follow-up to eb7f437, which made a failed copy report its statistics.

Two differences from GNU remained: when the write fails, the block that was
read is dropped instead of counted, so records in is one short; and the
message is IO error: <errno>, without the operation or the file.

$ dd if=/dev/zero of=/dev/full bs=1024 count=100
before: 0+0 records in / dd: IO error: No space left on device
after:  1+0 records in / dd: error writing '/dev/full': No space left on device
GNU:    1+0 records in / dd: error writing '/dev/full': No space left on device

dd_copy now returns UResult so the error carries the operation and the
file name; the existing ? paths keep the old generic context.

Refs #13092.

[codspeed-hq]

Merging this PR will improve performance by 4.99%

⚠️ 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

⚡ 1 improved benchmark
✅ 356 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	numfmt_stream_to_si_precision	348.8 ms	332.2 ms	+4.99%

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 igorgbr:dd/report-stats-on-io-error (e92919e) with main (ec8adb5)}

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:

Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.