Refactoring: move error logic into svelte component

src/UI/CustomThemeError.svelte

@@ -0,0 +1,31 @@
+<script lang="ts">
+  import ArrowDownTray from "@babeard/svelte-heroicons/mini/ArrowDownTray"
+  import { Utils } from "../Utils"
+
+  export let customDefinition: string
+  export let stack: string[]
+
+  function offerDefinitionForDownload() {
+    Utils.offerContentsAsDownloadableFile(
+      customDefinition,
+      "mapcomplete-theme.json",
+      { mimetype: "application/json" }
+    )
+  }
+
+</script>
+
+<div class="flex flex-col w-full h-full m-8 justify-center items-center">
+
+  <h1>Something went wrong</h1>
+  <div class="alert">{stack[0]}</div>
+  {#each stack.slice(1) as stck}
+    <div>{stck}</div>
+  {/each}
+  {#if customDefinition}
+    <button on:click={() => offerDefinitionForDownload()}>
+      <ArrowDownTray class="w-16 h-16" />
+      Download the theme definition file
+    </button>
+  {/if}
+</div>