Improve some `post_status`-related documentation.

Props ericlewis.
See #30230.

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Display error message when Media Library upload fails.

Props gcorne.
Fixes #29891.

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Check `$_current_page` before using it in `Walker_Page->start_el()`.

Props jeremyfelt.
Fixes #26901.

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All duplicate slugs across different post types.

Adds unit test.

Props mboynes, nacin.
Fixes #18962.

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Allow `get_pages()`, with `child_of` passed to it, to work with interrupted hierarchies.

Adds unit test.
Fixes #18962.

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