fix: add a warning when the misuse of reset in an error:boundary causes an error to be thrown when flushing the boundary content

[raythurnvoid]

Add warning message when situations like #16134 happen.

The user reporting the issue is misusing reset by calling it before the problematic template is rendered, reset "pauses" the error boundary and this allows the error to bubble up.

This adds a warning message when this happens instructing the user to fix the problematic app state before calling reset or to use reset when the user clicks a button in the fallback UI.

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Tests and linting

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

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16171-svelte.vercel.app/

[changeset-bot]

🦋 Changeset detected

Latest commit: f686128

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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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16171

[Rich-Harris]

Thank you. The behaviour of boundaries seems just wrong here — an error that occurs during reset should not escape the current boundary; that's deeply unhelpful. Surprised that never surfaced until now. Working on it locally, will push to this PR.

Checking that reset isn't called during onerror is a good call though, will keep that part of it