postgres cluster: Crash of master crashes the replicas too
This is harmless.
That message is sent by the server to the client when a process has crashed and the backend is about to die (quickdie in postgres.c) so that crash recovery can begin.
You are only seeing the message sent by the primary server to the WAL receiver. Note the WARNING: this is not even an error. This only gets logged because log_min_messages on the standby is set to warning or lower.
The standby server keeps running – as you see, it catches up from the archive while the primary server recovers. As soon as it has read all archives and the primary is up again, it will reconnect and continue streaming.