The "Contact your system administrator" isn't very helpful.
Not that the wording of that message would mean anything useful
to the average user either...

Also, remove showing error message because `ssh_session_is_known_server`
doesn't set any errors. Additionally, don't prepend SHA256 as
`ssh_get_fingerprint_hash` in its documentation says "the output is
prepended by the hash-type."

While at it, change the message box to use "Continue Anyway"
rather than an "Are you sure?" Yes/No question.