Add logging when password was not complex enough

[tflidd]

Is it possible to log events in the nextcloud.log when a user or an admin tried to change a password that is not complex enough. In some cases, there is no proper error message on the web-interface (partly due to a false nginx configuration but that's not for everybody: nextcloud/server#7619).

A dedicated error message in the nextcloud.log would help because we are asking systematically for the nextcloud.log when people report problems.