[feat] allow handleError to return a promise

[dummdidumm]

Closes #7756

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🦋 Changeset detected

Latest commit: b0cef89a1ee29d44c4934f699f350e2c41b6918e

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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@sveltejs/kit	Patch

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