warn users that --export-secret-ssh-key option doesn't work

[jp9zs1y8 at anonaddy.me]

users can't export ed25519 gpg keys to ssh format right now: https://lists.gnupg.org/pipermail/gnupg-users/2017-May/058172.html 
--export-secret-ssh-key implies this functionality, but it was confirmed that option doesn't work: https://dev.gnupg.org/T5840

yet as of gpg --version
gpg (GnuPG) 2.4.7
when you try to use this option it doesn't tell you that it doesn't work:
gpg --export-secret-ssh-key <KEYGRIP>
gpg: key "<KEYGRIP>" not found: Not a Keygrip

i think we should add warning to avoid confusion: --export-secret-ssh-key not implemented yet

best,
jp9zs1y8