Add exception to publish confirm context

The exception field is populated only in case of client errors. The
status is "rejected" for these cases.

Fixes #154

Author: acogoluegnes

src/main/java/com/rabbitmq/client/amqp/Consumer.java

@@ -153,7 +153,7 @@ interface Context {
    * the messages.
    *
    * <p>Only "simple" (not batch) message contexts can be added to a batch context. Calling {@link
-   * Context#batch()} on a batch context returns the instance itself.
+   * Context#batch(int)} on a batch context returns the instance itself.
    *
    * @see <a
    *     href="https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-transport-v1.0-os.html#type-disposition">AMQP

src/main/java/com/rabbitmq/client/amqp/Publisher.java

@@ -92,6 +92,13 @@ interface Context {
      * @see <a href="https://www.rabbitmq.com/docs/amqp#outcomes">AMQP Outcomes</a>
      */
     Status status();
+
+    /**
+     * Failure cause (null, unless for some cases of {@link Status#REJECTED}).
+     *
+     * @return failure cause, null if no cause for failure
+     */
+    Throwable failureCause();
   }
 
   /**

src/main/java/com/rabbitmq/client/amqp/impl/AmqpPublisher.java

@@ -131,7 +131,7 @@ public void publish(Message message, Callback callback) {
               } else {
                 status = Status.REJECTED;
               }
-              DefaultContext defaultContext = new DefaultContext(message, status);
+              DefaultContext defaultContext = new DefaultContext(message, status, ex);
               this.metricsCollector.publishDisposition(mapToPublishDisposition(status));
               callback.handle(defaultContext);
               return null;
@@ -227,10 +227,12 @@ private static class DefaultContext implements Publisher.Context {
 
     private final Message message;
     private final Status status;
+    private final Throwable failureCause;
 
-    private DefaultContext(Message message, Status status) {
+    private DefaultContext(Message message, Status status, Throwable failureCause) {
       this.message = message;
       this.status = status;
+      this.failureCause = failureCause;
     }
 
     @Override
@@ -242,6 +244,11 @@ public Message message() {
     public Status status() {
       return this.status;
     }
+
+    @Override
+    public Throwable failureCause() {
+      return this.failureCause;
+    }
   }
 
   Long id() {