Add custom error page.

src/routes/__error.svelte

+<script context="module" lang="ts">
+  export function load({ error, status }) {
+    return {
+      props: {
+        title: `${status}: ${error.message}`,
+      },
+    }
+  }
+</script>
+
+<script>
+  import { page, session } from "$app/stores"
+
+  export let title
+</script>
+
+<svelte:head>
+  <title>Erreur | {$session.title}</title>
+</svelte:head>
+
+<main>
+  <h1>{title}</h1>
+</main>