Change log level on write and flush failure to warn

This commit changes the log level on a write and flush failure to warn
as this is not necessarily an Elasticsearch problem but more likely
indicative of an infrastructure problem.

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

@@ -322,7 +322,7 @@ public class Netty4Transport extends TcpTransport<Channel> {
             } else {
                 final Throwable cause = f.cause();
                 Netty4Utils.maybeDie(cause);
-                logger.error("write and flush on the network layer failed", cause);
+                logger.warn("write and flush on the network layer failed", cause);
                 assert cause instanceof Exception;
                 listener.onFailure((Exception) cause);
             }