Raise exception if nested file will never be found

It is possible to give autoload an invalid path, and that issue will
never be found until the associated constant is vivified.  This causes
issues where people forget to remove the require_nested calls when they
remove the files.  Now, it's not possible to forget, because an
exception will be raised as soon as the parent file is loaded.

lib/extensions/require_nested.rb

@@ -6,6 +6,7 @@ def require_nested(name)
     filename = "#{self}::#{name}".underscore
     filename = name.to_s.underscore if self == Object
     if Rails.application.config.cache_classes
+      raise LoadError, "No such file to load -- #{filename}" unless ActiveSupport::Dependencies.search_for_file(filename)
       autoload name, filename
     else
       require_dependency filename