REST API: After [39302], clarify `author_ip` parameter in error message.

Properties of objects should not be translated, and therefore are pulled out of the translation strings.

Props ramiy.
Fixes #38822.
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git-svn-id: https://core.svn.wordpress.org/trunk@39246 1a063a9b-81f0-0310-95a4-ce76da25c4cd