#391 Post-Python 3: Publisher Merge Results doesn't check publisher existence

[Ahasuerus]

Publisher Merge Results (edit/pv_merge.cgi) doesn't check publisher existence. If one of the to-be-merged publisher records was deleted between the time the submitter ran Publisher Search and the time s/he clicked "Merge Selected records", Publisher Merge Results will create an invalid submission. It's not a major issue because the submission review page will force a Hard reject error, but it would be better to catch it earlier.