Adds custom 404 Page

As mentioned in #11, we need a 404 page this adds one by just adding a +error.svelte page. No need for fallback or any other changes.

Fixes #11

src/routes/+error.svelte

+<div>
+  <h1 class="mt-8 text-4xl font-bold">Woops! - 404 PageNotFound</h1>
+  <div class="max-w-page mx-auto">
+    <p class="py-4 text-xl">The page you're looking for couldn't be found.</p>
+  </div>
+</div>