Improve logging of failable deliverables
    
    The pungi.global.log should show subvariant for each failed deliverable
    (if available). When the compose finishes, there is a new log file in
    logs/global/deliverables.json containing details about all deliverables
    as triples of (variant, arch, subvariant).
    
     * `required` lists all deliverables that can not fail
     * `attempted` lists all failable deliverables that were started
     * `failed` is a subset of `attempted` and only contains deliverables
       that failed
    
    If the compose fails, the lists may be incomplete.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>