Add a migration guide for this PR.

Author: andriyDev

release-content/migration-guides/duplicate_subasset_label_error.md

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+---
+title: Adding duplicate subassets names now returns an error.
+pull_requests: []
+---
+
+Previously, when adding a subasset in a loader through `LoadContext::add_labeled_asset` (and
+friends), adding the same subasset name multiple times would silently replace the asset (and which
+one would be used was undefined). Now these functions return an error if the subasset label was
+already present. You can use `?` to return the error from `AssetLoader`s.