REST API: Clarify parameters when used in error strings.

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

Props ocean90, ramiy, danielbachhuber.
Fixes #38822.
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git-svn-id: https://core.svn.wordpress.org/trunk@39238 1a063a9b-81f0-0310-95a4-ce76da25c4cd