In show_message(), only print error data if it is a string.

see #25576, [25775].

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Only add error data to the files_not_writable WP_Error if the install already has [25775] and [25796] applied.

see #22704.

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Avoid our pre-flight writable checks if get_core_checksums() doesn't exist yet.

Thus, it will not work for 3.6 => 3.7, only 3.7+.

see #22704.

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Remove accidental debug cruft in [25780]. see #22704.

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Use correct variable. see #22704.

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Significantly simplify get_core_checksums(), as the caching and chunking was causing too much grief.

Make sure we only do our pre-flight is_writable check when the file exists.

see #18201. see #22704.

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Twenty Fourteen: better logic for featured image HTML output, and add fallback message for focusable anchor elements, for accessibility. Fixes #25325.
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