Response sent in chunked transfer encoding and indicating errors happening after some data has already been sent

Once you have started sending the HTTP headers to the client, you can't send anything else. You have to finish sending the response you intended to send, ie the chunked data and associated headers. If an error occurs midway through that, there is no way to report that error to the client. All you can do is close the connection. Either the client does not receive all of the headers, or it does not receive the terminating 0-length chunk at the end of the response. Either way is enough for the client to know that the server encountered an error during sending.