app/views/application/_error_messages.html.erb
<% if object.errors.any? %>
<div id="error_explanation">
<div class="alert alert-danger">
<h2 class="h4"><%= pluralize(object.errors.size, "error") %>:</h2>
<ul class="list-styled">
<% object.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
</div>
<% end %>