REST API: Improve formatting of failed validation errors.

If a validation_callback returns a WP_Error it should give the same response format as if it returned `false`. This makes programmatically reading the validation errors better.

Props bradyvercher for initial patch.
Fixes #35028.

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git-svn-id: https://core.svn.wordpress.org/trunk@35854 1a063a9b-81f0-0310-95a4-ce76da25c4cd