docs: error boundary tutorial

apps/svelte.dev/content/tutorial/02-advanced-svelte/07-special-elements/07-svelte-boundary/+assets/app-a/src/lib/App.svelte

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+<script>
+	import FlakyComponent from './FlakyComponent.svelte';
+</script>
+
+<FlakyComponent />

apps/svelte.dev/content/tutorial/02-advanced-svelte/07-special-elements/07-svelte-boundary/+assets/app-a/src/lib/FlakyComponent.svelte

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+<script>
+	let mouse = $state({ x: 0, y: 0 });
+</script>
+
+<svelte:window
+	onmousemove={(e) => {
+		mouse.x = e.clientX;
+		mouse.y = e.clientY;
+	}}
+/>
+
+<p>{mouse.x}x{mouse.y}</p>
+
+<button onclick={() => mouse = null}>
+	whatever you do, don't click this button
+</button>

apps/svelte.dev/content/tutorial/02-advanced-svelte/07-special-elements/07-svelte-boundary/+assets/app-b/src/lib/App.svelte

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+<script>
+	import FlakyComponent from './FlakyComponent.svelte';
+</script>
+
+
+<svelte:boundary onerror={(e) => console.error(e)}>
+	<FlakyComponent />
+
+	{#snippet failed(error, reset)}
+		<p>Oops! {error.message}</p>
+		<button onclick={reset}>Reset</button>
+	{/snippet}
+</svelte:boundary>

apps/svelte.dev/content/tutorial/02-advanced-svelte/07-special-elements/07-svelte-boundary/index.md

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+---
+title: <svelte:boundary>
+---
+
+To prevent errors from leaving your app in a broken state, you can contain them inside an _error boundary_ using the `<svelte:boundary>` element.
+
+In this example, `<FlakyComponent>` contains a bug — clicking the button will set `mouse` to `null`, meaning that the `{mouse.x}` and `{mouse.y}` expressions in the template will fail to render.
+
+In an ideal world we'd simply fix the bug. But that's not always an option — sometimes the component belongs to someone else, and sometimes you just need to guard against the unexpected. Begin by wrapping `<FlakyComponent />` with `<svelte:boundary>`:
+
+```svelte
+<!--- file: App.svelte --->
++++<svelte:boundary>+++
+	<FlakyComponent />
++++</svelte:boundary>+++
+```
+
+So far, nothing has changed, because the boundary doesn't specify a handler. Add a `failed` [snippet](snippets-and-render-tags) to provide some UI to show when an error occurs:
+
+```svelte
+<!--- file: App.svelte --->
+<svelte:boundary>
+	<FlakyComponent />
+
++++	{#snippet failed(error)}
+		<p>Oops! {error.message}</p>
+	{/snippet}+++
+</svelte:boundary>
+```
+
+Now, when we click the button, the contents of the boundary are replaced with the snippet. We can attempt to reset things by making use of the second argument passed to `failed`:
+
+```svelte
+<!--- file: App.svelte --->
+<svelte:boundary>
+	<FlakyComponent />
+
+	{#snippet failed(error+++, reset+++)}
+		<p>Oops! {error.message}</p>
+		+++<button onclick={reset}>Reset</button>+++
+	{/snippet}
+</svelte:boundary>
+```
+
+We can also specify an `onerror` handler, which is called with the same arguments passed to the `failed` snippet:
+
+```svelte
+<!--- file: App.svelte --->
+<svelte:boundary +++onerror={(e) => console.error(e)}+++>
+	<FlakyComponent />
+
+	{#snippet failed(error, reset)}
+		<p>Oops! {error.message}</p>
+		<button onclick={reset}>Reset</button>
+	{/snippet}
+</svelte:boundary>
+```
+
+This is useful for sending information about the error to a reporting service, or adding UI outside the error boundary itself.