NotionApiRateLimitException to expose Retry-After value

Author: KoditkarVedant

Src/Notion.Client/NotionApiException.cs

@@ -5,7 +5,7 @@
 namespace Notion.Client
 {
     [SuppressMessage("ReSharper", "MemberCanBePrivate.Global")]
-    public sealed class NotionApiException : Exception
+    public class NotionApiException : Exception
     {
         public NotionApiException(HttpStatusCode statusCode, NotionAPIErrorCode? notionAPIErrorCode, string message)
             : this(statusCode, notionAPIErrorCode, message, null)
@@ -29,4 +29,19 @@ private NotionApiException(
 
         public HttpStatusCode StatusCode { get; }
     }
+
+    public sealed class NotionApiRateLimitException : NotionApiException
+    {
+        public TimeSpan? RetryAfter { get; }
+
+        public NotionApiRateLimitException(
+            HttpStatusCode statusCode,
+            NotionAPIErrorCode? notionAPIErrorCode,
+            string message,
+            TimeSpan? retryAfter)
+            : base(statusCode, notionAPIErrorCode, message)
+        {
+            RetryAfter = retryAfter;
+        }
+    }
 }

Src/Notion.Client/RestClient/RestClient.cs

@@ -112,6 +112,17 @@ private static async Task<Exception> BuildException(HttpResponseMessage response
                 try
                 {
                     errorResponse = JsonConvert.DeserializeObject<NotionApiErrorResponse>(errorBody);
+
+                    if (errorResponse.ErrorCode == NotionAPIErrorCode.RateLimited)
+                    {
+                        var retryAfter = response.Headers.RetryAfter.Delta;
+                        return new NotionApiRateLimitException(
+                            response.StatusCode,
+                            errorResponse.ErrorCode,
+                            errorResponse.Message,
+                            retryAfter
+                        );
+                    }
                 }
                 catch (Exception ex)
                 {