Make it easier to file distro bugs

Discover currently tells users to file bug reports against their distro in a few places, but doesn't tell users how to do so, and in any event is often is bit cagey about what exactly the error is.

As a result, we still often get bug reports against Discover that are distro issues which we can't do anything about. This wastes developer and bug triager time in KDE, frustrates users by giving them the run-around, and makes Discover look bad in cases where the problem is someone else's fault.

This commit attempts to improve that situation by making the error messages clearer and also adding a big "Report this issue" button that users can click on which will take them to the bug report URL for their distro, not for Discover itself.

BUG: 442785 FIXED-IN: 5.24

The UI now, with a simulated error message since I use a distro that doesn't ever have update errors 😎 :

cc @teams/vdg @apol