Fix authentication

Summary:
According to the CUPS code (https://github.com/apple/cups/blob/master/cups/auth.c#L105) cupsDoAuthentication returns 0 on success and -1 on error. The code was assuming the opposite

Now the CUPS password dialog is shown when needed. This is quite often, e.g directly at startup due to getServerSettings() so it might be worth checking if that's really necessary.

BUG: 408512

Test Plan: Open print-manager with a user that is not in a CUPS system group. Privileged operations work after entering a password

Reviewers: dantti, broulik

Reviewed By: dantti

Subscribers: ognarb, rdieter, kde-utils-devel

Differential Revision: https://phabricator.kde.org/D23403