Warn instead of a fatal error when ciphers do not exist.

[Robert]

Hi,

Not sure if this is the right place to post.
When one of the ciphers listed under "Ciphers" in sshd_config does not
exist, it refuses to start. This can happen when admins set their
preferred ciphers and an update removed some ciphers. Admins can be
locked out of remote machines after a reboot.
I think it is better just ignore non-exist ciphers and generates
warnings. Only exit when there is no ciphers available.

Regards,
Robert