Add these lines above the yield block in your application layout as suggested in one comment.
# In application.html.erb
<% flash.each do |name, msg| %>
# New code (allow for flash elements to be arrays)
<% if msg.class == Array %>
<% msg.each do |message| %>
<%= content_tag :div, message, :id => "flash_#{name}" %>
<% end %>
<% else %>
# old code
<%= content_tag :div, msg, :id => "flash_#{name}" %>
<% end %> #don't forget the extra end
<% end %>
Just add this code in the controller action, where you want to show the error messages,
flash[:notice] = flash[:notice].to_a.concat resource.errors.full_messages
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