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 a crash any more.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7543