Changed default CustomCommandHandler

While throwing an IllegalStateException is great to signal developers they need to register a default handler, it's a dangerous default for all those who don't need one. A log entry on 'debug' level should suffice.

core/src/main/java/ch/dissem/bitmessage/BitmessageContext.java

@@ -435,8 +435,8 @@ public class BitmessageContext {
                 customCommandHandler = new CustomCommandHandler() {
                     @Override
                     public MessagePayload handle(CustomMessage request) {
-                        throw new IllegalStateException(
-                            "Received custom request, but no custom command handler configured.");
+                        LOG.debug("Received custom request, but no custom command handler configured.");
+                        return null;
                     }
                 };
             }