Error on servers with pure SFTP access

[momolog]

There are servers that allow SFTP only but not SSH shell access, e.g. my web server at my hosting provider df.eu (domainfactory).

Under these circumstances,
https://github.com/alexeypetrushin/vos/blob/master/lib/vos/drivers/ssh.rb#L107
will fail and
https://github.com/alexeypetrushin/vos/blob/master/lib/vos/drivers/ssh.rb#L109
raises an uncaught error.

This behaviour makes it impossible to use the driver for servers that are set up like this.
It would be better to let home return '' then instead of raising the error.