Port forwarding bug fix: we were unable to handle receiving

CHANNEL_OPEN_FAILURE messages, which occur when the remote side is
unable to open a forwarded network connection we have requested. (It
seems they _don't_ show up if you get something mundane like
Connection Refused - the channel is cheerfully opened and
immediately slammed shut - but they do if you try to connect to a
host that doesn't even exist. Try forwarding a port to
frogwibbler:4800 and see what you get.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@1213 cda61777-01e9-0310-a592-d414129be87e