cutter: Don't set error on concurrent video update

If two cutters modify a video at the same time, youtube may respond with a 409 to one of them.
We want to treat that as a retryable error, and ideally let another cutter finish it instead.

cutter/cutter/main.py

@@ -768,7 +768,14 @@ class VideoUpdater(object):
 						else:
 							self.logger.exception("Failed to update video")
 
-						self.mark_errored(job.id, "Failed to update video: {}".format(ex))
+						# Explicitly retryable errors aren't problems
+						if isinstance(ex, UploadError) and ex.retryable:
+							self.logger.warning("Retryable error when updating video", exc_info=True)
+							# By giving up without marking as done or errored, another cutter should get it.
+							# Or we'll get it next loop.
+						else:
+							self.mark_errored(job.id, "Failed to update video: {}".format(ex))
+
 						continue
 
 					marked = self.mark_done(job, updates)

cutter/cutter/upload_backends.py

@@ -254,6 +254,8 @@ class Youtube(UploadBackend):
 			},
 			metric_name='update_video',
 		)
+		if resp.status_code == 409:
+			raise UploadError("Multiple updates to same video, got 409: {}".format(resp.text), retryable=True)
 		resp.raise_for_status()
 
 	def set_thumbnail(self, video_id, thumbnail):