RPC server: Do not log exceptions with 4xx HTTP status codes

They are client errors, therefore it does not make sense to log them
as exceptions (which is only useful for debugging the server).

swh/core/api/__init__.py

@@ -429,10 +429,9 @@ def error_handler(exception, encoder, status_code=500):
     on the client side.
 
     """
-    logging.exception(exception)
-
     status_class = status_code // 100
     if status_class == 5:
+        logging.exception(exception)
         sentry_sdk.capture_exception(exception)
 
     response = encoder(exception_to_dict(exception))