ssh: Improve the error message for unreadable known_hosts
The new message describes several ways that the
SSH_KNOWN_HOSTS_UNKNOWN error can be produced. After this change for
example:
$ ./nbdkit ssh host=localhost /tmp/foo known-hosts=/nosuchfile \
--run 'nbdinfo --size $uri'
nbdkit: ssh[1]: error: either the host key is unknown, you must use ssh first and accept the host key; or the known_hosts file (usually ~/.ssh/known_hosts) could not be opened, check that the file exists and file permissions
Reported-by: Jonathon Jongsma
Fixes: https://gitlab.com/nbdkit/nbdkit/-/issues/10