Some improvements for custom message handling

domain/src/main/java/ch/dissem/bitmessage/entity/CustomMessage.java

@@ -16,10 +16,7 @@
 
 package ch.dissem.bitmessage.entity;
 
-import java.io.ByteArrayOutputStream;
-import java.io.IOException;
-import java.io.InputStream;
-import java.io.OutputStream;
+import java.io.*;
 
 import static ch.dissem.bitmessage.utils.Decode.bytes;
 
@@ -65,4 +62,12 @@ public class CustomMessage implements MessagePayload {
                     "Programmer: did you forget to override #write()?");
         }
     }
+
+    public static CustomMessage error(String message) {
+        try {
+            return new CustomMessage(("ERROR\n" + message).getBytes("UTF-8"));
+        } catch (UnsupportedEncodingException e) {
+            throw new RuntimeException(e);
+        }
+    }
 }

domain/src/main/java/ch/dissem/bitmessage/ports/NetworkHandler.java

@@ -16,6 +16,7 @@
 
 package ch.dissem.bitmessage.ports;
 
+import ch.dissem.bitmessage.entity.CustomMessage;
 import ch.dissem.bitmessage.entity.ObjectMessage;
 import ch.dissem.bitmessage.entity.valueobject.InventoryVector;
 import ch.dissem.bitmessage.utils.Property;
@@ -34,7 +35,18 @@ public interface NetworkHandler {
      * An implementation should disconnect if either the timeout is reached or the returned thread is interrupted.
      * </p>
      */
-    Future<?> synchronize(InetAddress trustedHost, int port, MessageListener listener, long timeoutInSeconds);
+    Future<?> synchronize(InetAddress server, int port, MessageListener listener, long timeoutInSeconds);
+
+    /**
+     * Send a custom message to a specific node (that should implement handling for this message type) and returns
+     * the response, which in turn is expected to be a {@link CustomMessage}.
+     *
+     * @param server  the node's address
+     * @param port    the node's port
+     * @param request the request
+     * @return the response
+     */
+    CustomMessage send(InetAddress server, int port, CustomMessage request);
 
     /**
      * Start a full network node, accepting incoming connections and relaying objects.