Fwupd: Don't consider "systemd service is masked" to be an error

This is typically something done deliberately; if someone does this, they don't want to be greeted by an error message every time they launch Discover. Instead, just silently invalidate the Fwupd backend.

Why would someone mask the service rather than removing Discover's backend for it, or uninstalling Fwupd? They may be on an immutable distro, or a distro that makes those packages mandatory.