Add GatewayTimeout to defined errors (#243)

Author: ariarijp

twitter_ads/error.py

@@ -96,12 +96,17 @@ def retry_after(self):
         return self._retry_after
 
 
+class GatewayTimeout(Error):
+    """Gateway Timeout (504)."""
+
+
 ERRORS = {
     400: BadRequest,
     401: NotAuthorized,
     403: Forbidden,
     404: NotFound,
     429: RateLimit,
     500: ServerError,
-    503: ServiceUnavailable
+    503: ServiceUnavailable,
+    504: GatewayTimeout
 }