Rails helper to render the errors or a form as flash messages.

application_helper.rb

  def flash_errors_for(model)
    return unless model.errors.any?
    
    messages = [I18n.t("errors.messages.not_saved",
                       count: model.errors.size,
                       resource: model.model_name)]
    # Sanitize any errors
    messages << model.errors.to_a.map(&:html_safe)
    # assign to alert, mark as safe
    flash.now.alert = messages.join("<br/>").html_safe
  end

passwords_new.html.erb

<h2><%= t("forgot_password") %></h2>
<%- flash_errors_for(@pw_reset_instructions) %>

<%= simple_form_for(@pw_reset_instructions,
                    as: :user,
                    url: passwords_path) do |f| %>
  <%= f.input :email %>
  <%= f.button :submit, t("send_password_instructions") %>
<% end %>