Make initial setup much clearer
There are a lot of ways that initial Samba sharing setup can go wrong if the distro hasn't set up everything perfectly out of the box. Right now the wizard shows some error messages when problems are encountered, but the logic fails to account for the full set of things that can go wrong during initial setup, and as a result are sometimes displays misleading error messages that can send the user down the wrong path.
This commit expands the set of conditions that are checked for, and offers more verbose and actionable error messages specific to each of them.
To accomplish this, the code is refactored to move nearly all of the logic to the backend, with the QML side simply presenting the information for whatever error was encountered.
The existing code already had logic for showing a friendly button to fix the "you're not a member of the right group" error condition, which is perserved here. TODOs are added in the place where other similar actions to fix the other errors could be defined. That can be done in another MR.
BUG: 425202 FIXED-IN: 22.12