Studio: move some validation from throwing errors in LayerConfig.ts into errors in Validation

src/UI/Studio/ErrorIndicatorForRegion.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+  import EditLayerState from "./EditLayerState";
+  import { ExclamationIcon } from "@rgossiaux/svelte-heroicons/solid";
+
+  export let firstPaths: Set<string>;
+  export let state: EditLayerState;
+  let messagesCount = state.messages.map(msgs => msgs.filter(msg => {
+    const pth = msg.context.path
+    return firstPaths.has(pth[0]) || (pth.length > 1 && firstPaths.has(pth[1]));
+  }).length);
+
+</script>
+
+{#if $messagesCount > 0}
+  <span class="alert flex w-min">
+    <ExclamationIcon class="w-6 h-6" />
+    {$messagesCount}
+  </span>
+{/if}