Panic if ssh known_hosts file does not exist

[Tepri]

Walker currently panics with the following error if the ssh known_hosts file does not exist:

panic: open /home/tepri/.ssh/known_hosts: no such file or directory [recovered] panic: closure error: unexpected panic caught: open /home/tepri/.ssh/known_hosts: no such file or directory

I store my file in a different location (~/.config/ssh/known_hosts), some users may not use ssh at all.

[Andrej Benz]

fixed by e5b5806.