Fix bug in !956

Closes #464 (closed)

Edit: The diff looks large, but it's because it's also undoing the revert in !961 (merged). The diff between this and what was merged in !956 (merged) is

$ git diff 12c4f9ea8f781fff7aced5840a2e1e4c8d506d29 3a25ac79ff661a4f86ed5f2df2ef967f99712822
diff --git a/gwcelery/tasks/orchestrator.py b/gwcelery/tasks/orchestrator.py
index 9da22bb..3dd2950 100644
--- a/gwcelery/tasks/orchestrator.py
+++ b/gwcelery/tasks/orchestrator.py
@@ -7,7 +7,7 @@ candidates.
 import json
 import re
 
-from celery import group
+from celery import chain, group
 
 from ..import app
 from . import alerts
@@ -570,7 +570,8 @@ def earlywarning_preliminary_alert(event, alert, annotation_prefix='',
     # Download files from events and upload to superevent with relevant
     # annotations. Pass file contents down the chain so alerts task doesn't
     # need to download them again.
-    canvas = (
+    # Note: this is explicitly made a chain to fix an issue described in #464.
+    canvas = chain(
         group(
             gracedb.download.si(skymap_filename, preferred_event_id)
             |