Give a better error message if a PuTTY private key file has a version
number we don't understand. It's nicer to report 'format too new' than
'not a PuTTY key file'.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9763 cda61777-01e9-0310-a592-d414129be87e