cli: Fix exit code for building images
    
    When spin-livemedia, image-build etc. start a task and watch it, they
    never return a correct exit value. Pungi relies on the exit code to
    determine if the task failed or not.
    
    Fixes: https://pagure.io/koji/issue/555
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>