add tests, move CustomError to common, fix casting

common/script/api-v3/errors.js

@@ -0,0 +1,9 @@
+import CustomError from './customError';
+
+export class NotAuthorized extends CustomError {
+  constructor (customMessage) {
+    super();
+    this.name = this.constructor.name;
+    this.message = customMessage || 'Not authorized.';
+  }
+}