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>