Fail on error in JavascriptPackageTest for UJS

Prior to this commit, if `app/javascript/rails-ujs/index.js`
contained a syntax error, `JavascriptPackageTest` would still pass
because `system "yarn build"` would simply return `false` and the
compiled output would not change.  This commit adds `exception: true` to
the `system` call so that an error will be raised if `yarn build` fails.

Author: jonathanhefner

actionview/test/javascript_package_test.rb

@@ -2,15 +2,15 @@
 
 class JavascriptPackageTest < ActiveSupport::TestCase
   def test_compiled_code_is_in_sync_with_source_code
-    assert_no_changes -> {
-      %w[
-        app/assets/javascripts/rails-ujs.js
-        app/assets/javascripts/rails-ujs.esm.js
-      ].map { |compiled_file|
-        File.read(File.expand_path("../#{compiled_file}", __dir__))
-      }
-    } do
-      system "yarn build"
+    compiled_files = %w[
+      app/assets/javascripts/rails-ujs.js
+      app/assets/javascripts/rails-ujs.esm.js
+    ].map do |file|
+      Pathname(file).expand_path("#{__dir__}/..")
+    end
+
+    assert_no_changes -> { compiled_files.map(&:read) } do
+      system "yarn build", exception: true
     end
   end
 end