Added original exception to FailureCallback action (#449)

Author: Towmeykaw

sample/Serilog.Sinks.Elasticsearch.Sample/Program.cs

@@ -40,7 +40,7 @@ static void Main(string[] args)
                     NumberOfShards = 2,
                     //BufferBaseFilename = "./buffer",
                    // RegisterTemplateFailure = RegisterTemplateRecovery.FailSink,
-                    FailureCallback = e => Console.WriteLine("Unable to submit event " + e.MessageTemplate),
+                    FailureCallback = (e, ex) => Console.WriteLine("Unable to submit event " + e.MessageTemplate),
                     EmitEventFailure = EmitEventFailureHandling.WriteToSelfLog |
                                        EmitEventFailureHandling.WriteToFailureSink |
                                        EmitEventFailureHandling.RaiseCallback,

src/Serilog.Sinks.Elasticsearch/Sinks/ElasticSearch/ElasticSearchSink.cs

@@ -142,7 +142,7 @@ private void HandleException(Exception ex, IEnumerable<LogEvent> events)
                 {
                     foreach (var e in events)
                     {
-                        _state.Options.FailureCallback(e);
+                        _state.Options.FailureCallback(e, ex);
                     }
                 }
                 catch (Exception exCallback)
@@ -233,7 +233,7 @@ private void HandleResponse(IEnumerable<LogEvent> events, DynamicResponse result
                             // Send to a failure callback
                             try
                             {
-                                _state.Options.FailureCallback(e);
+                                _state.Options.FailureCallback(e, null);
                             }
                             catch (Exception ex)
                             {

src/Serilog.Sinks.Elasticsearch/Sinks/ElasticSearch/ElasticsearchSinkOptions.cs

@@ -246,7 +246,7 @@ public class ElasticsearchSinkOptions
         /// A callback which can be used to handle logevents which are not submitted to Elasticsearch
         /// like when it is unable to accept the events. This is optional and depends on the EmitEventFailure setting.
         /// </summary>
-        public Action<LogEvent> FailureCallback { get; set; }
+        public Action<LogEvent, Exception> FailureCallback { get; set; }
 
         /// <summary>
         /// The maximum number of events that will be held in-memory while waiting to ship them to