add source name to error message

weed/storage/backend/s3_backend/s3_download.go

@@ -47,7 +47,7 @@ func downloadFromS3(sess s3iface.S3API, destFileName string, sourceBucket string
 		Key:    aws.String(sourceKey),
 	})
 	if err != nil {
-		return fileSize, fmt.Errorf("failed to download file %s: %v", destFileName, err)
+		return fileSize, fmt.Errorf("failed to download /buckets/%s%s to %s: %v", sourceBucket, sourceKey, destFileName, err)
 	}
 
 	glog.V(1).Infof("downloaded file %s\n", destFileName)