Fix quota error formatting in case of NoneType
[pithos] / other /
drwxr-xr-x   ..
-rwxr-xr-x 3691 migrate-data
-rwxr-xr-x 13561 migrate-db
-rwxr-xr-x 4140 migrate-users
-rw-r--r-- 3125 migrate.py
-rw-r--r-- 3133 stats-calculator.sql