add extensible-custom-error to app

packages/app/package.json

@@ -101,6 +101,7 @@
     "express-session": "^1.16.1",
     "express-validator": "^6.1.1",
     "express-webpack-assets": "^0.1.0",
+    "extensible-custom-error": "^0.0.7",
     "got": "^8.3.2",
     "graceful-fs": "^4.1.11",
     "helmet": "^4.6.0",