Added error logging and error messaging response when errors occur

graphql-java-kickstart/src/main/java/graphql/kickstart/execution/subscriptions/DefaultSubscriptionSession.java

@@ -53,8 +53,8 @@ public void sendDataMessage(String id, Object payload) {
   }
 
   @Override
-  public void sendErrorMessage(String id) {
-
+  public void sendErrorMessage(String id, Object payload) {
+    send(mapper.serialize(payload));
   }
 
   @Override

graphql-java-kickstart/src/main/java/graphql/kickstart/execution/subscriptions/SessionSubscriber.java

@@ -1,8 +1,13 @@
 package graphql.kickstart.execution.subscriptions;
 
 import graphql.ExecutionResult;
+
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
+
+import graphql.GraphQLError;
+import graphql.kickstart.execution.error.GenericGraphQLError;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.reactivestreams.Subscriber;
@@ -29,15 +34,20 @@ public void onSubscribe(Subscription subscription) {
   public void onNext(ExecutionResult executionResult) {
     Map<String, Object> result = new HashMap<>();
     result.put("data", executionResult.getData());
+
     session.sendDataMessage(id, result);
     subscriptionReference.get().request(1);
   }
 
   @Override
   public void onError(Throwable throwable) {
     log.error("Subscription error", throwable);
+
+    Map<String, Object> payload = new HashMap<>();
+    payload.put("errors", Collections.singletonList(new GenericGraphQLError(throwable.getMessage())));
+
     session.unsubscribe(id);
-    session.sendErrorMessage(id);
+    session.sendErrorMessage(id, payload);
   }
 
   @Override

graphql-java-kickstart/src/main/java/graphql/kickstart/execution/subscriptions/SubscriptionSession.java

@@ -19,7 +19,7 @@ public interface SubscriptionSession {
 
   void sendDataMessage(String id, Object payload);
 
-  void sendErrorMessage(String id);
+  void sendErrorMessage(String id, Object payload);
 
   void sendCompleteMessage(String id);
 

graphql-java-kickstart/src/main/java/graphql/kickstart/execution/subscriptions/apollo/ApolloSubscriptionSession.java

@@ -18,8 +18,8 @@ public void sendDataMessage(String id, Object payload) {
   }
 
   @Override
-  public void sendErrorMessage(String id) {
-    sendMessage(new OperationMessage(Type.GQL_ERROR, id, null));
+  public void sendErrorMessage(String id, Object payload) {
+    sendMessage(new OperationMessage(Type.GQL_ERROR, id, payload));
   }
 
   @Override

graphql-java-kickstart/src/main/java/graphql/kickstart/execution/subscriptions/apollo/SubscriptionConnectionInitCommand.java

@@ -19,8 +19,8 @@ public void apply(SubscriptionSession session, OperationMessage message) {
       connectionListeners.forEach(it -> it.onConnect(session, message));
       session.sendMessage(new OperationMessage(Type.GQL_CONNECTION_ACK, message.getId(), null));
     } catch (Throwable t) {
-      session.sendMessage(
-          new OperationMessage(Type.GQL_CONNECTION_ERROR, message.getId(), t.getMessage()));
+      log.error("Cannot initialize subscription command '{}'", message, t);
+      session.sendMessage(new OperationMessage(Type.GQL_CONNECTION_ERROR, message.getId(), t.getMessage()));
     }
   }
 

graphql-java-servlet/src/main/java/graphql/kickstart/servlet/subscriptions/WebSocketSendSubscriber.java

@@ -35,7 +35,7 @@ public void onNext(String message) {
 
   @Override
   public void onError(Throwable t) {
-
+    log.error("WebSocket error", t);
   }
 
   @Override