Netty: make exceptionCaught method protected

The exceptionCaught method had default access, which imposes a requirement
for subclasses that need to override this method to be in a specific package. This
change simply makes the method protected, which removes the package requirement.

src/main/java/org/elasticsearch/transport/netty/NettyTransport.java

@@ -605,7 +605,7 @@ public class NettyTransport extends AbstractLifecycleComponent<Transport> implem
         return this.boundAddress;
     }
 
-    void exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) throws Exception {
+    protected void exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) throws Exception {
         if (!lifecycle.started()) {
             // ignore
             return;