Log cause when a write and flush fails

This commit logs the cause of a write and flush operation on the network
layer that failed.

modules/transport-netty4/src/main/java/org/elasticsearch/transport/netty4/Netty4Transport.java

@@ -321,6 +321,7 @@ public class Netty4Transport extends TcpTransport<Channel> {
                 listener.onResponse(channel);
             } else {
                 final Throwable cause = f.cause();
+                logger.error("write and flush on the network layer failed", cause);
                 Netty4Utils.maybeDie(cause);
                 assert cause instanceof Exception;
                 listener.onFailure((Exception) cause);