fserrors: add "server closed idle connection" to retriable errors

This seems to be related to this go issue: golang/go#19943

See: https://forum.rclone.org/t/copy-from-dropbox-to-google-drive-yields-failed-to-copy-failed-to-open-source-object-server-closed-idle-connection-error/7460

fs/fserrors/error.go

@@ -233,6 +233,7 @@ var retriableErrorStrings = []string{
 	"unexpected EOF reading trailer",   // net/http/transfer.go
 	"transport connection broken",      // net/http/transport.go
 	"http: ContentLength=",             // net/http/transfer.go
+	"server closed idle connection",    // net/http/transport.go
 }
 
 // Errors which indicate networking errors which should be retried