#環境設定
ruby '2.5.1'
rails '~> 5.2.4', '>= 5.2.4.1'
mysql2 '>= 0.4.4', '< 0.6.0'
devise 4.7.1

#質問内容
表題と通り、ユーザー登録が項目未記入などの理由で登録失敗した際に、
登録画面にリダイレクトしたいと思います。
登録成功時は「after_sign_up_path_for(resource)」を使ってトップページの飛ばしているのですが、
失敗時はどのようにすれば良いでしょうか。

#app/controllers/users/registorations_controller.rb


class Users::RegistrationsController < Devise::RegistrationsController
  # before_action :configure_sign_up_params, only: [:create]
  # before_action :configure_account_update_params, only: [:update]

  def prefecture
    @prefectures = Prefecture.all
  end

  # def create
  #   if @user.empty?
  #     # Handle a successful save.
  #   else
  #     redirect_to signup_path
  #   end
  # end

  # GET /resource/sign_up
  # def new
  #   super
  # end
  
  # POST /resource
  # def create
  #   super
  # end

  # GET /resource/edit
  # def edit
  #   super
  # end

  # PUT /resource
  # def update
  #   super
  # end

  # DELETE /resource
  # def destroy
  #   super
  # end

  # GET /resource/cancel
  # Forces the session data which is usually expired after sign
  # in to be expired now. This is useful if the user wants to
  # cancel oauth signing in/up in the middle of the process,
  # removing all OAuth session data.
  # def cancel
  #   super
  # end

  def detail
    @user = User.find_by(id: params[:id])
  end



  # protected

  # If you have extra params to permit, append them to the sanitizer.
  # def configure_sign_up_params
  #   devise_parameter_sanitizer.permit(:sign_up, keys: [:attribute])
  # end

  # If you have extra params to permit, append them to the sanitizer.
  # def configure_account_update_params
  #   devise_parameter_sanitizer.permit(:account_update, keys: [:attribute])
  # end

  # The path used after sign up.
  def after_sign_up_path_for(resource)
    "/"
  end

  # The path used after sign up for inactive accounts.
  # def after_inactive_sign_up_path_for(resource)
  #   redirect_to signup_path
  # end
end

#config/routes.rb

Rails.application.routes.draw do
  resources :prefectures
  
  root to: 'top#index'
  resources :items, only: [:new]

  devise_for :users, :controllers => {
    :registrations => 'users/registrations',
    :sessions => 'users/sessions'   
  } 
  resources :orders, only: [:new]
  devise_scope :user do
    get "user/:id", :to => "users/registrations#detail"
    get "signup", :to => "users/registrations#new"
    get "login", :to => "users/sessions#new"
    get "logout", :to => "users/sessions#destroy"
  end
end

app/views/devise/registorations/new.html.haml

.registrationsNew
  %h2< 会員登録
  = form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f|
    -#= render "devise/shared/error_messages", resource: resource
    .request 全て必須事項です。
    %span.nickname
      = f.label :nickname
      = f.text_field :nickname
    %br/
    %span.email
      = f.label :email
      = f.email_field :email, autofocus: true, autocomplete: "email"
    %br/
    %span.password
      = f.label :password
      - if @minimum_password_length
        %em
          (#{@minimum_password_length} 文字以上の英数字)
      = f.password_field :password, autocomplete: "new-password"
    %br/
    %span.passwordConfirm
      = f.label :password_confirmation
      = f.password_field :password_confirmation, autocomplete: "new-password"
    %br/
    %span.lastName
      = f.label :last_name
      = f.text_field :last_name, autofocus: true, autocomplete: "last_name"
    
    %span.firstName
      = f.label :first_name 
      = f.text_field :first_name, autofocus: true, autocomplete: "first_name"
    %br/
    %span.lastNameKana
      = f.label :last_name_kana
      = f.text_field :last_name_kana, autofocus: true, autocomplete: "last_name_kana"
    %span.firstNameKana
      = f.label :first_name_kana
      = f.text_field :first_name_kana, autofocus: true, autocomplete: "first_name_kana"
    %br/
    %span.tel
      = f.label :tel
      = f.telephone_field :tel, autofocus: true, autocomplete: "tel"
    %br/
    %span.zipCode
      = f.label :zip_code
      = f.text_field :zip_code, autofocus: true, autocomplete: "zip_code"
    %br/
    %span.prefecture
      = f.label :prefecture
      = f.collection_select :prefecture_id, Prefecture.group(:prefecture).order(id: :asc), :id, :prefecture, {prompt: ""}
    %br/
    %span.address
      = f.label :address
      = f.text_field :address, autofocus: true, autocomplete: "address"
    %br/
    %span.birth 生年月日（西暦）
    %span.birthYear
      = f.label :birth_year
      = f.number_field :birth_year, autofocus: true, autocomplete: "birth_year", min:1900, max:2020,value:2000
    %span.birthMonth
      = f.label :birth_month
      = f.number_field :birth_month, autofocus: true, autocomplete: "birth_month", min:1, max:12,value:1
    %span.birthDay
      = f.label :birth_day
      = f.number_field :birth_day, autofocus: true, autocomplete: "birth_day", min:1, max:31,value:1
    .actions
      = f.submit "登録"
  = render "devise/shared/links"