WIP Add default support for Propshaft::MissingAssetError

TODO: tests...currently the test app requires sprockets-rails - should
we create a test app that uses propshaft?

Author: rmacklin

lib/importmap/engine.rb

@@ -9,6 +9,7 @@ class Engine < ::Rails::Engine
     config.importmap.sweep_cache = Rails.env.development? || Rails.env.test?
     config.importmap.rescuable_asset_errors = [
       defined?(Sprockets::Rails) && Sprockets::Rails::Helper::AssetNotFound,
+      defined?(Propshaft) && Propshaft::MissingAssetError,
     ].compact
 
     config.autoload_once_paths = %W( #{root}/app/helpers )