Show only one dialog when failing to import keys

GnuPG-Bug-ID: 6749

The dialog result dialog shouldn't be shown if all jobs failed, as it's duplicate information. This is made somewhat more complex since when importing certificates, both an smime and a pgp import job is started. The pgp job will "succeed" (with no data / certificates). this is thus filtered out by the check whether any certificates were even considered for importing.