app/views/shared/_errors.html.erb
<% if model.errors.any? %>
<div class="alert alert-danger">
<h4><%= pluralize(model.errors.count, "error") %> prohibited this task from being saved:</h4>
<ul>
<% model.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>