Moves current API under JSON namespace

lib/API/JSON/Error.php

@@ -0,0 +1,16 @@
+<?php
+namespace MailPoet\API\JSON;
+
+if(!defined('ABSPATH')) exit;
+
+final class Error {
+  const UNKNOWN = 'unknown';
+  const BAD_REQUEST = 'bad_request';
+  const UNAUTHORIZED = 'unauthorized';
+  const FORBIDDEN = 'forbidden';
+  const NOT_FOUND = 'not_found';
+
+  private function __construct() {
+
+  }
+}