Suppress error for existing directories

[saschabratton]

Proposed solution for https://github.com/gtg092x/gulp-sftp/issues/34

Doesn't actually check if directories exist or not, so the mkdir command is still sent regardless, but simply suppresses the error if the directory exists.

[criztovyl]

For me this does not work, err does not seem to have the code attribute.

[saschabratton]

Thanks @criztovyl. I'll take a look if I can make a better "solution" for more operating systems. What OS are you using?

[criztovyl]

I'm on Debian Stretch.