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

src/applications/harbormaster/storage/HarbormasterBuildable.php

@@ -34,6 +34,7 @@ final class HarbormasterBuildable extends HarbormasterDAO
     $buildable = id(new HarbormasterBuildableQuery())
       ->setViewer($actor)
       ->withBuildablePHIDs(array($buildable_object_phid))
+      ->setLimit(1)
       ->executeOne();
     if ($buildable) {
       return $buildable;