Provide a clearer error message when qemu-block-curl is missing #2360
Description
When qemu-block-curl is missing installing with sourceType='url' fails with a fairly big error message on RHEL.
Can we detect if qemu-block-curl is available via a libvirt/qemu API and then maybe disable the option? Otherwise can we catch the error and provide a better error message?
This is related to cockpit-project/bots#2538 of which the issue is that we lack a soft-dependency on qemu-block-curl.