pkgset: Handle exceptions in pkgset threads
    
    There are two thread pools for making package sets. If Pungi is being
    terminated by external event and the exception is handled in the first
    thread, the second one never gets to the `stop` method and the process
    keeps hanging.
    
    This patch should make sure that `stop()` is called on both pools.
    
    JIRA: RHELCMP-2459
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>