app/views/content/form/_errors.html.erb
<% if content.errors.any? %>
<div class="card-panel red lighten-4">
This <%= content.class.to_s.downcase %> couldn't be saved because of the following errors:
<% content.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</div>
<% end %>