Warn instead of raising on missing manifest.js

With a pristine (`rails new`) Rails 8.0 app, the `app/assets/config/manifest.js` file
is no longer automatically generated. For engines still using sprockets,
that is not a huge issue, because they have and add their own manifest
files for their assets to the list of precompiled assets.

However, scripts like the Solidus bin/sandbox script[1] first run `rails
new`, then add the needed engines to the sandbox's `Gemfile`, then run
`rails db:migrate` and their respective install generators. With Rails
8, these scripts now fail before anything is possible in Ruby, because
the initializer of `Sprockets::Railtie` will raise an error on startup.

This commit changes the behavior of the Railtie to issue a deprecation
warning instead of raising an Exception.

[1] https://github.com/solidusio/solidus/blob/main/bin/sandbox#L44-L99

Author: mamhoff

lib/sprockets/railtie.rb

@@ -60,21 +60,6 @@ module Sprockets
   class Railtie < ::Rails::Railtie
     include Sprockets::Rails::Utils
 
-    class ManifestNeededError < StandardError
-      def initialize
-        msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +
-        "But did not, please create this file and use it to link any assets that need\n" +
-        "to be rendered by your app:\n\n" +
-        "Example:\n" +
-        "  //= link_tree ../images\n"  +
-        "  //= link_directory ../javascripts .js\n" +
-        "  //= link_directory ../stylesheets .css\n"  +
-        "and restart your server\n\n" +
-        "For more information see: https://github.com/rails/sprockets/blob/070fc01947c111d35bb4c836e9bb71962a8e0595/UPGRADING.md#manifestjs"
-        super msg
-      end
-    end
-
     LOOSE_APP_ASSETS = lambda do |logical_path, filename|
       filename.start_with?(::Rails.root.join("app/assets").to_s) &&
       !['.js', '.css', ''].include?(File.extname(logical_path))
@@ -103,8 +88,20 @@ def configure(&block)
 
     initializer :set_default_precompile do |app|
       if using_sprockets4?
-        raise ManifestNeededError unless ::Rails.root.join("app/assets/config/manifest.js").exist?
-        app.config.assets.precompile += %w( manifest.js )
+        if ::Rails.root.join("app/assets/config/manifest.js").exist?
+          app.config.assets.precompile += %w( manifest.js )
+        else
+          msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +
+          "But did not, please create this file and use it to link any assets that need\n" +
+          "to be rendered by your app:\n\n" +
+          "Example:\n" +
+          "  //= link_tree ../images\n"  +
+          "  //= link_directory ../javascripts .js\n" +
+          "  //= link_directory ../stylesheets .css\n"  +
+          "and restart your server\n\n" +
+          "For more information see: https://github.com/rails/sprockets/blob/070fc01947c111d35bb4c836e9bb71962a8e0595/UPGRADING.md#manifestjs"
+          Sprockets::Rails.deprecator.warn msg
+        end
       else
         app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
       end