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).

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.
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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint