Fail prematurely on setup if test conditions are not met

[SamuAlfageme]

This will prevent false positives (due to e.g. missing quota, server down, etc.)

e.g. the first op from reset_owncloud_account (webdav_delete) for 3 users:

curl -X DELETE https://<user>:<password>@<host>/remote.php/webdav/home/<dir> -vvv
[...]
< HTTP/1.1 500 Internal Server Error
< Server: nginx/1.6.2
< Date: Tue, 29 Jun 2021 07:55:10 GMT
< Content-Type: text/html
< Content-Length: 0
< Connection: keep-alive