add failed call back.

src/DotNetCore.CAP/Processor/IProcessor.FailedJob.cs

@@ -53,14 +53,32 @@ namespace DotNetCore.CAP.Processor
         private async Task ProcessPublishedAsync(IStorageConnection connection, ProcessingContext context)
         {
             var messages = await connection.GetFailedPublishedMessages();
+            var hasException = false;
+
             foreach (var message in messages)
             {
+                if (!hasException)
+                {
+                    try
+                    {
+                        _options.FailedCallback?.Invoke(Models.MessageType.Publish, message.Name, message.Content);
+
+                    }
+                    catch (Exception ex)
+                    {
+                        hasException = true;
+                        _logger.LogWarning("Failed call-back method raised an exception:" + ex.Message);
+                    }
+                }
+
                 using (var transaction = connection.CreateTransaction())
                 {
                     _stateChanger.ChangeState(message, new EnqueuedState(), transaction);
                     await transaction.CommitAsync();
                 }
+
                 context.ThrowIfStopping();
+
                 await context.WaitAsync(_delay);
             }
         }
@@ -68,14 +86,32 @@ namespace DotNetCore.CAP.Processor
         private async Task ProcessReceivededAsync(IStorageConnection connection, ProcessingContext context)
         {
             var messages = await connection.GetFailedReceviedMessages();
+            var hasException = false;
+
             foreach (var message in messages)
             {
+                if (!hasException)
+                {
+                    try
+                    {
+                        _options.FailedCallback?.Invoke(Models.MessageType.Subscribe, message.Name, message.Content);
+
+                    }
+                    catch (Exception ex)
+                    {
+                        hasException = true;
+                        _logger.LogWarning("Failed call-back method raised an exception:" + ex.Message);
+                    }
+                }
+
                 using (var transaction = connection.CreateTransaction())
                 {
                     _stateChanger.ChangeState(message, new EnqueuedState(), transaction);
                     await transaction.CommitAsync();
                 }
+
                 context.ThrowIfStopping();
+
                 await context.WaitAsync(_delay);
             }
         }