Retry HTTP 408 Request Timeout errors

Fixes #469.

gwcelery/tasks/gracedb.py

@@ -31,7 +31,7 @@ def catch_retryable_http_errors(f):
         try:
             return f(*args, **kwargs)
         except HTTPError as e:
-            if e.response.status_code in {429, 502, 503, 504}:
+            if e.response.status_code in {408, 429, 502, 503, 504}:
                 raise RetryableHTTPError(
                     *e.args, request=e.request, response=e.response)
             else: