main: Do not exit on distro page failure, but show dialog
When failing to load a distro page, don't exit. This means that users with invalid distro pages won't be prevented from seeing the Welcome Center. Instead, we show a dialog which makes the user aware of an issue and advises them to report it to their distributor.
For testing on Qt 6, cherry-pick !79 (merged)'s first commit.
Edited by Oliver Beard