[core][js] misc cleanup/validation

lib/internals.js

@@ -33,6 +33,15 @@ export default {
       return `The [${appName}] firebase app has not been initialized!`;
     },
 
+    /**
+     * @param optName
+     * @return {string}
+     * @constructor
+     */
+    ERROR_MISSING_OPT(optName) {
+      return `Failed to initialize app. FirebaseOptions missing or invalid '${optName}' property.`;
+    },
+
     /**
      * @return {string}
      */