Print password error message on stderr

The password prompt itself is already printed on stderr.

changelog/unreleased/pull-3716

@@ -0,0 +1,7 @@
+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