Fatal Error: Call to undefined function in a drush backup module
The function variable_get doesn't exist in Drupal 8. I suggest just removing ~/.drush/backups/pantheon/20161102190143/modules/xmlsitemap/xmlsitemap.drush.inc or moving ~/.drush/backups to ~/.drushbackups if you can control this. 

Your problem is probably caused by the fact that you are putting the backup inside the ~/.drush directory and Drush is parsing code/files in that directory.