app/views/layouts/_model_error.html.erb
<% if model_error.errors? %>
<div class='usa-alert usa-alert-error'>
<div class="usa-alert-body">
<h3 class="usa-alert-heading"><%= model_error.heading %></h3>
<div class='usa-alert-body'>
<ul>
<% model_error.messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
</div>
</div>
<% end %>