[SFTP] Allow continuing when host identity has changed
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.