Bugfix: Print "wrong password" error on stderr

If a wrong password was entered, the error message was printed on stdout and
not on stderr as intended. This has been fixed.

https://github.com/restic/restic/pull/3716
https://forum.restic.net/t/should-error-messages-end-up-in-output-file-when-redirecting-dump-to-stdout/4965
