Keep the stacktrace of exceptions in async calls

Ensure that the initial callstack isn't lost when exceptions are
re-thrown by DispatchAsync.

Issue #1399

Author: vbfox

src/Nest/ElasticClient.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Reflection;
 using System.Threading.Tasks;
 using Elasticsearch.Net;
 using Elasticsearch.Net.Connection;
@@ -125,6 +126,21 @@ Func<D, D> selector
 			return this.DispatchAsync<D, Q, R, I>(descriptor, dispatch);
 		}
 
+		private static readonly Lazy<MethodInfo> preserveStackTraceMethodInfo = new Lazy<MethodInfo>(() => 
+			typeof(Exception).GetMethod("InternalPreserveStackTrace", BindingFlags.Instance | BindingFlags.NonPublic)
+		);
+
+		private static void RethrowKeepingStackTrace(Exception exception)
+		{
+			// In .Net 4.5 it would be simple : ExceptionDispatchInfo.Capture(exception).Throw();
+			// But as NEST target .Net 4.0 the old internal method must be used
+			if (preserveStackTraceMethodInfo.Value != null)
+			{
+				preserveStackTraceMethodInfo.Value.Invoke(exception, null);
+			}
+			throw exception;
+		}
+
 		private Task<I> DispatchAsync<D, Q, R, I>(
 			D descriptor
 			, Func<ElasticsearchPathInfo<Q>, D, Task<ElasticsearchResponse<R>>> dispatch
@@ -142,12 +158,13 @@ D descriptor
 					{
 						var mr = r.Exception.InnerException as MaxRetryException;
 						if (mr != null)
-							throw mr;
+							RethrowKeepingStackTrace(mr);
 
 						var ae = r.Exception.Flatten();
 						if (ae.InnerException != null)
-							throw ae.InnerException;
-						throw ae;
+							RethrowKeepingStackTrace(ae.InnerException);
+
+						RethrowKeepingStackTrace(ae);
 					}
 					return ResultsSelector<D, Q, R>(r.Result, descriptor);
 				});