views: add error pages for 403, 404 and 419

views/errors/419.php

@@ -0,0 +1,10 @@
+<section class="block">
+    <div class="wrap" style="max-width:520px;text-align:center">
+        <div class="empty">
+            <div style="font-family:var(--font-body);font-size:3.4rem;font-weight:700;color:var(--accent)">419</div>
+            <h2>Your session expired</h2>
+            <p class="muted">The security token on that form was missing or stale. Please go back and try again.</p>
+            <a class="btn btn--primary mt" href="/">Back to the shop</a>
+        </div>
+    </div>
+</section>