mostrar un resumen de los errores

app/views/posts/_form.haml

@@ -3,6 +3,19 @@
     %h4= t('.errors.title')
     %p= t('.errors.help')
 
+    %ul
+      - post.errors.each do |attribute, errors|
+        - if errors.size > 1
+          %li
+            %strong= post_label_t attribute, post: post
+            %ul
+              - errors.each do |error|
+                %li= error
+        - else
+          %li
+            %strong= post_label_t attribute, post: post
+            = errors.first
+
 -# TODO: habilitar form_for
 :ruby
   if post.new?