Adding more API tests

src/exceptions.js

@@ -15,6 +15,14 @@ export class ValidationError extends Error {
 
   #errorCode;
 
+  /**
+   * Getter for #errorCode
+   * @return {number}
+   */
+  get errorCode() {
+    return this.#errorCode;
+  }
+
   /**
    * Trying to override the default Error message
    * @return {string}