ipa-pki-retrieve-key: ensure we do not crash
    
    If ipa-pki-retrieve-key fails for some reason (which may be a
    "legitimate" reason, e.g. the server it is attempting to contact
    being offline), the program terminates with an uncaught exception,
    resulting in crash report.
    
    Catch all exceptions; if an exception gets raised, report the
    traceback and exit with nonzero status.
    
    Fixes: https://pagure.io/freeipa/issue/7115
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>