app/views/devise/shared/_error_messages.html.erb
<% if resource.errors.any? %>
<div id="error_explanation">
<h2>
<%= t("errors.messages.not_saved",
count: resource.errors.count,
resource: resource.class.model_name.human.downcase)
%>
</h2>
<ul>
<% resource.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>