[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 by documentation says "the output is prepended by the hash-type."

@sitter @fvogt @dfaure

No idea why the icons on the buttons are missing but that has been like this in sftp since I can remember