return exit code 11 if the repository could not be locked

cmd/restic/cmd_migrate.go

@@ -24,6 +24,7 @@ EXIT STATUS
 
 Exit status is 0 if the command was successful.
 Exit status is 1 if there was any error.
+Exit status is 11 if the repository is already locked.
 `,
 	DisableAutoGenTag: true,
 	RunE: func(cmd *cobra.Command, args []string) error {