app/views/proposals/_validation_errors.html.erb
<%= form_for(@proposal) do |f| %>
<% if @proposal.errors.any? %>
<div class="alert alert-danger">
<p class="lead"><%= I18n.t 'activerecord.errors.template.header', count: @proposal.errors.size %></p>
<% @proposal.errors.messages.values.each do |msg| %>
<% msg.each do |text| %>
<p><%= text %></p>
<% end %>
<% end %>
</div>
<% end %>
<% end %>