Improve error message when deploying backup w/ no config

rattail_fabric2/backup.py

@@ -78,7 +78,8 @@ def deploy_backup_app(c, deploy, envname, mkvirtualenv=True, user='rattail',
             if deploy.local_exists(path):
                 config = path
             else:
-                raise ValueError("Must provide config path for backup app")
+                raise ValueError("Config file not found for backup; "
+                                 "please add {} to your deploy folder".format(path))
 
     if install_borg:
         borg.install_dependencies(c)