(#11423) Clearer error message about duplicate imported resources.

When multiple nodes export a resource with a duplicate type and title, and you import both, the error message misleadingly tells you that this was a *local* duplication.

This improves the error message to cover all the bases, which is suboptimal - it should tell you where the duplicate comes from - but at least doesn't lead to a wild goose hunt for the *local* duplicate that doesn't exist.