uucore: keep the usage hint when a caret replaced the message

[sylvestre]

A quieted UUsageError became a bare ExitCode, whose usage() is false, so
stdbuf and cut printed "Try '... --help'" on a pipe but not on a terminal.

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[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will not alter performance

✅ 28 untouched benchmarks
⏩ 380 skipped benchmarks¹

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_{Comparing caret-usage-hint (839e910) with caret-join (601f0f6)²}

Footnotes

1. 380 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. ↩

2. No successful run was found on caret-join (f2987b5) during the generation of this report, so 2da090c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

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