Customize: 'Changeset already published' message isn't always accurate

[dlh]

When opening a changeset on customize.php where the changeset has the status of publish or trash, the error message is This changeset has already been published and cannot be further modified.

Although published changesets are trashed by default, so too are discarded changesets as of 4.9. The error message isn't quite accurate in the case of a discarded changeset, though.

One way to change the string so it applies in both cases might be shortening it to This changeset cannot be further modified or This set of changes cannot be further modified.

[westonruter]

In 42150:

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

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

[johnbillion]

This went into trunk but not into the 4.9 branch, but wasn't re-opened with fixed-major.

[dd32]

Bumping to 4.9.3 due to 4.9.2s release

[westonruter]

In 42535:

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

Amends [38810].
Props dlh.
Fixes #42497 for 4.9 branch.