discourse/app/views/embed/embed_error.html.erb

<header class='discourse' data-embed-status='error'>
  <h3><%= t 'embed.error' %></h3>
  <%= link_to(image_tag(SiteSetting.logo_url, class: 'logo'), Discourse.base_url) %>
  <div class='clearfix'></div>
</header>
<%- if @show_reason %>
  <div class='embed-error'>
    <%- if @hosts.present? %>
      <p><%= t 'embed.referer' %><code><%= request.referer %></code></p>
      <p><%= t 'embed.mismatch' %></p>

      <ul>
        <%- @hosts.each do |eh| %>
          <li>
            <%= eh.host %><%- if eh.path_whitelist.present? %><%= eh.path_whitelist %><% end %>
          </li>
        <%- end %>
      </ul>
    <%- else %>
      <p><%= t 'embed.no_hosts' %></p>
    <%- end %>

    <p><%= link_to(t('embed.configure'), @setup_url, target: '_blank') %></p>
  </div>
<%- end %>