cutter: Set an error message to warn humans when it discards a candidate due to holes

This prevents videos being stuck in EDITED with no visible problem when
they contain holes, but is likely to false positive sometimes.

This is fine though, as it's just a human-readable warning and
it will be cleared as soon as any node accepts the row to be cut.

cutter/cutter/main.py

@@ -130,19 +130,9 @@ class Cutter(object):
 			# Shuffle the list so that (most of the time) we don't try to claim the same one as other nodes
 			random.shuffle(candidates)
 			for candidate in candidates:
-				try:
-					segments = self.check_candidate(candidate)
-				except ContainsHoles:
-					# TODO metric
-					self.logger.info("Ignoring candidate {} due to holes".format(format_job(candidate)))
-					continue # bad candidate, let someone else take it or just try again later
-				except Exception as e:
-					# Unknown error. This is either a problem with us, or a problem with the candidate
-					# (or most likely a problem with us that is only triggered by this candidate).
-					# In this case we would rather stay running so other jobs can continue to work if possible.
-					# But to give at least some feedback, we set the error message on the job
-					# if it isn't already.
-					self.logger.exception("Failed to check candidate {}, setting error on row".format(format_job(candidate)))
+
+				def set_error(error):
+					"""Common code for the two paths below, for setting an error on the row for humans to see"""
 					try:
 						# Since this error message is just for humans, we don't go to too large
 						# a length to prevent it being put on the row if the row has changed.
@@ -152,7 +142,7 @@ class Cutter(object):
 							UPDATE events
 							SET error = %s
 							WHERE id = %s AND state = 'EDITED' AND error IS NULL
-						""", id=candidate.id, error='{}: Error while checking candidate: {}'.format(self.name, e))
+						""", id=candidate.id, error=error)
 					except Exception:
 						self.logger.exception("Failed to set error for candidate {}, ignoring".format(format_job(candidate)))
 						self.refresh_conn()
@@ -160,12 +150,37 @@ class Cutter(object):
 						if result.rowcount > 0:
 							assert result.rowcount == 1
 							self.logger.info("Set error for candidate {}".format(format_job(candidate)))
+
+				try:
+					segments = self.check_candidate(candidate)
+				except ContainsHoles:
+					# TODO metric
+					self.logger.info("Ignoring candidate {} due to holes".format(format_job(candidate)))
+					set_error(
+						"Node {} does not have all the video needed to cut this row. "
+						"This may just be because it's too recent and the video hasn't been downloaded yet. "
+						"However, it might also mean that there is a 'hole' of missing video, perhaps "
+						"because the stream went down or due to downloader issues. If you know why this "
+						"is happening and want to cut the video anyway, re-edit with the 'Allow Holes' option set."
+					.format(self.name))
+					continue # bad candidate, let someone else take it or just try again later
+				except Exception as e:
+					# Unknown error. This is either a problem with us, or a problem with the candidate
+					# (or most likely a problem with us that is only triggered by this candidate).
+					# In this case we would rather stay running so other jobs can continue to work if possible.
+					# But to give at least some feedback, we set the error message on the job
+					# if it isn't already.
+					self.logger.exception("Failed to check candidate {}, setting error on row".format(format_job(candidate)))
+					set_error('{}: Error while checking candidate: {}'.format(self.name, e))
 					self.wait(self.ERROR_RETRY_INTERVAL)
 					continue
+
 				if all(segment is None for segment in segments):
 					self.logger.info("Ignoring candidate {} as we have no segments".format(format_job(candidate)))
 					continue
+
 				return CutJob(segments=segments, **candidate._asdict())
+
 			# No candidates
 			self.wait(self.NO_CANDIDATES_RETRY_INTERVAL)