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

@@ -55,6 +55,9 @@ class Engine < ::Rails::Engine
       if defined?(Sprockets::Rails)
         app.config.importmap.rescuable_asset_errors << Sprockets::Rails::Helper::AssetNotFound
       end
+      if defined?(Propshaft)
+        app.config.importmap.rescuable_asset_errors << Propshaft::MissingAssetError
+      end
     end
   end
 end