This is what is logged whenever the replaying of a query fails because it cannot find a valid candidate.

Write query received from user@127.0.0.1. ‘delayed_retry_timeout’ exceeded before a primary could be found. Closing client connection.

The problem is that when this error was logged, there was a valid server with the Master label available. If readwritesplit detects that a server with the correct state would be available but it cannot be used for some other reason, it should be included in the error.