cutter: Handle case where we are told to stop while looking for candidates

Previously, it would return None and things would break. Now the None is handled
correctly, and is documented.

cutter/cutter/main.py

@@ -85,6 +85,9 @@ class Cutter(object):
 		# any unhandled errors will cause the process to restart and clean up as per rollback_all_owned().
 		while not self.stop.is_set():
 			job = self.find_candidate()
+			if not job:
+				# find_candidate() returning None means we're stopping
+				continue
 			try:
 				self.claim_job(job)
 			except CandidateGone:
@@ -99,7 +102,7 @@ class Cutter(object):
 	def find_candidate(self):
 		"""List EDITED events and find one at random which we have all segments for
 		(or for which allow_holes is true), returning a CutJob.
-		Polls until one is available.
+		Polls until one is available, or we are stopping (in which case it returns None)
 		"""
 		while not self.stop.is_set():
 			try: