pkgset: Report better error when module is missing an arch
    
    Pungi expects each module to be built for all architectures by default.
    Unless the module is filtered out, missing metadata for a particular
    arch would cause it to crash with a incomprehensible error message. This
    should make it a little better.
    
    Relates: https://pagure.io/releng/failed-composes/issue/3889
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>