fix notion api error message parsing

Author: KoditkarVedant

Src/Notion.Client/NotionApiErrorResponse.cs

@@ -1,8 +1,13 @@
-namespace Notion.Client
+using Newtonsoft.Json;
+
+namespace Notion.Client
 {
     class NotionApiErrorResponse
     {
+        [JsonProperty("code")]
         public NotionAPIErrorCode ErrorCode { get; set; }
+
+        [JsonProperty("message")]
         public string Message { get; set; }
     }
 }