Return error if ssh-config command fails
Return error if ssh-config command fails

Update error detection to return an error if the process fails instead
of testing for content in stderr.

hashicorp/packer,rnaveiras/packer,mitchellh/packer,rnaveiras/packer,hashicorp/packer,ricardclau/packer,pecigonzalo/packer,mitchellh/packer,ricardclau/packer,ricardclau/packer,mitchellh/packer,ricardclau/packer,pecigonzalo/packer,mitchellh/packer,pecigonzalo/packer,pecigonzalo/packer,ricardclau/packer,hashicorp/packer,hashicorp/packer,mitchellh/packer,rnaveiras/packer,rnaveiras/packer,ricardclau/packer,rnaveiras/packer,rnaveiras/packer