mkfifo: point at the failing part of an invalid mode

[sylvestre]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 347 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing mode-diag-3-mkfifo (78158a8) with mode-diag-2-mkdir (369d97f)}

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. ↩

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 147.54 MB (+76 KB, +0.05%)

Significant per-binary changes:
  mkfifo     1.08 MB ->    1.16 MB  (+76 KB, +6.86%)

[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/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Skip an intermittent issue tests/tail/tail-n0f (was skipped on 'main', now failing)