[example] Do not log errors when operation was cancelled

In case when HTTP connection was closed and the operation was still waiting to be run. Then OperationCancelled error is returned by processor.Run.

Author: elgopher

Diff for: _example/http/http.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"strconv"
 
+	"github.com/elgopher/batch"
 	"github.com/elgopher/batch/_example/train"
 )
 
@@ -50,12 +51,16 @@ func bookHandler(trainService TrainService) func(http.ResponseWriter, *http.Requ
 			return
 		}
 
+		if errors.Is(err, batch.OperationCancelled) {
+			// context.Context was cancelled, so the operation
+			// this could happen when connection was closed or request was cancelled (http/2)
+			return
+		}
+
 		if err != nil {
 			writer.WriteHeader(http.StatusInternalServerError)
 			fmt.Println("internal server error:", err)
 			return
 		}
-
-		writer.WriteHeader(http.StatusOK)
 	}
 }