app/views/shared/_alerts.html.erb
<% flash.each do |name, msg| %>
<%
# https://guides.rubyonrails.org/v4.1/upgrading_ruby_on_rails.html#flash-structure-changes
%>
<%= content_tag :div, :class => "alert #{["error", "alert"].include?(name.to_s) ? "alert-error" : "alert-success" }" do %>
<a class="close" data-dismiss="alert" href="#">×</a>
<%= msg %>
<% end %>
<% end %>