Allow more error page parameters to be customized to suit the error, and use that to show a custom, Arch-specific error message when no app backends are installed

Two commits which should not be squashed:

commit 1c677381de2ccb47552f368cf1f6b01ae46695e4 (HEAD -> more-custom-error-pages)
Author: Nate Graham <nate@kde.org>
Date:   Thu May 19 09:31:44 2022 -0600

    Show an Arch-specific message if no app backends are available
    
    Arch developers have repeatedly expressed a preference for Arch users to
    use the command line to manage software, rather than Discover. For this
    reason, they deliberately package Discover in such a way that you will
    have to go out of your way to figure out how to install an app backend.
    
    Unfortunately, this means that Discover looks broken if you install it
    with its default set of dependencies, because it won't be able to find
    any apps. Novice Arch users don't understand that this is a deliberate
    packaging choice in Arch, and instead blame Discover.
    
    To alleviate this, we have introduced an error page that has become
    ever mode specific over time about what exactly the problem is. Despite
    this, novice Arch users remain confused about it. And the text we show
    today is not really correct, as it implies that the issue is caused by
    a bug in Arch's packaging. But it is *not* a bug; Arch packagers
    package Discover this way deliberately, because they don't want Arch
    users to use Discover to get apps. They consider this to be an expert
    level feature that you shouldn't use unless you know what you're doing,
    which inherently conflicts with the UX that we want to offer in
    Discover.
    
    Accordingly, for Arch users, let's show an Arch-specific message that
    explains what's going on in clear and precise language, and offers an
    actionable path forward.

commit 0e26d22c34bff68d443c09e3b8aaa0154e86742b
Author: Nate Graham <nate@kde.org>
Date:   Thu May 19 09:29:19 2022 -0600

    Allow more error page parameters to be customized to suit the error
    
    Some errors might benefit from showing different explanatory text, or
    having the button look different or take the user to a different page.

With this MR applied, here's what will be seen by users of various distros who launch Discover without any app backends installed:

Arch Linux	Any other distro

@teams/vdg @teams/usability @arojas