fix: more accurate error message when creating orphan effects

[Rich-Harris]

This tightens up the error message you get when creating an orphan effect, and dries out the code a bit

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[changeset-bot]

🦋 Changeset detected

Latest commit: af6fb31

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Patch

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