Allow manifest.js to be placed anywhere.

lib/sprockets/railtie.rb

@@ -69,7 +69,7 @@ class Railtie < ::Rails::Railtie
 
     class ManifestNeededError < StandardError
       def initialize
-        msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +
+        msg = "Expected to find a manifest file in `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" +
@@ -103,7 +103,6 @@ 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 )
       else
         app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
@@ -212,6 +211,10 @@ def self.build_manifest(app)
         end
       end
 
+      if using_sprockets4?
+        raise ManifestNeededError if config.assets.find_asset('manifest.js').nil?
+      end
+
       app.assets_manifest = build_manifest(app)
 
       if config.assets.resolve_with.nil?