Customize: Improve accuracy of message displayed when accessing Customizer...

Customize: Improve accuracy of message displayed when accessing Customizer with a changeset that is already published or trashed.

Amends [38810].
Props dlh.
Fixes #42497.

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git-svn-id: https://core.svn.wordpress.org/trunk@41981 1a063a9b-81f0-0310-95a4-ce76da25c4cd