REST API: Display the actual conflicting value in addition to the taxonomy...

REST API: Display the actual conflicting value in addition to the taxonomy name in the `_doing_it_wrong()` message for a conflicting `rest_base` or `name` property.

Follow-up to [47037] and [47045].

Props TimothyBlynJacobs.
See #48401.
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git-svn-id: https://core.svn.wordpress.org/trunk@46846 1a063a9b-81f0-0310-95a4-ce76da25c4cd