BulkObservable should look at item.IsValid not response.IsValid

This also feeds the failing documents through the callback whether we
continue afterwards or not

(cherry picked from commit f7e3262)

Author: Mpdreamz

src/Nest/Document/Multiple/Bulk/BulkResponseItem/BulkResponseItemBase.cs

@@ -129,6 +129,7 @@ public bool IsValid
 			}
 		}
 
-		public override string ToString() => $"{Operation} returned {Status} _index: {Index} _type: {Type} _id: {Id} _version: {Version} error: {Error}";
+		public override string ToString() =>
+			$"{Operation} returned {Status} _index: {Index} _type: {Type} _id: {Id} _version: {Version} error: {Error}";
 	}
 }