Small JobQueueFederated performance tweaks

* Made "find non-empty queues" and isEmpty() checks start with the most
  weighted partitions first
* Cleaned up a few exception catches to use JobQueueError
* Added short-circuit for doGetSiblingQueuesWithJobs()

Change-Id: I4d96a8c61c6b1c41dbb44000ccda0fa05a418a09