#398 Post-Python 3: Title Merge errors out on a deleted parent title

[Ahasuerus]

Title Merge errors out on a deleted parent title. If an editor loads a "Title Merge Results" Web page with a "title_parent" value and selects that value and then, before clicking "Complete Merge", deletes the title_parent title record from the database, the next Web page will error out with an Apache error. This is a low probability scenario, but it still needs to be fixed.