R1:43847d6bd7bb

Prevent crashes when more than one buildable for an object exists

Summary: This prevents a crash in applying build plans when more than one buildable exists for the same object. It also adds a check into the "New Manual Build" page to ensure that users can't create a buildable for an object that already has one.

Test Plan: Tried to create a buildable for an object that already has one and a nice friendly error appeared. Applied a build plan to a buildable whose object has two buildables and didn't get…