Add custom error using standard Rails error Model

_error_messages.erb

<% if object and object.errors.any? %>
    <div class="error_explanation">
      <h2><%= pluralize(object.errors.count, "error") %> prohibited this <%= object.class.model_name.human.downcase %> from being saved:</h2>

      <ul>
        <% object.errors.full_messages.each do |msg| %>
            <li>* <%= msg %></li>
        <% end %>
      </ul>
    </div>
<% end %>

view.html.erb

<% @user.errors.add(:base, "either name or email must be present") %>
<%= render 'admin/shared/error_messages', object: @user %>