fold: continue processing files after an open error

[sylvestre]

Should make test fold/multiple-files.sh pass

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/fold/multiple-files is no longer failing!
Congrats! The gnu test tests/cut/bounded-memory is now passing!

[cakebaker]

I don't know if you have seen it but there is a conflict.

[codspeed-hq]

Merging this PR will not alter performance

✅ 323 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing sylvestre:fold-continue-after-missing-file (6ddf485) with main (962566b)}

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. ↩