fix: Don't destroy boundary contents on error unless the boundary is an error boundary

[elliott-with-the-longest-name-on-github]

Right now, if you wrap some content in a svelte:boundary and don't specify a failed snippet or onerror handler, an error inside that boundary will cause its contents to be destroyed. This is confusing behavior, as you could wrap your app in a svelte:boundary with a pending snippet and suddenly the error behavior would differ (as non-wrapped content is not destroyed on an error). This PR adjusts the logic such that when an error is thrown, as it traverses the boundary tree, it only causes the contents of a boundary to be removed if that boundary is an error boundary (i.e. it defines an onerror handler or a failed snippet).

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🦋 Changeset detected

Latest commit: cdd556b

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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