Make trying to resend a collection mail a warning instead of an asser…

…tion

Summary:
Tasks are expected to be re-run occasionally if they fail to ack for some reason,
so issuing a warning and terminating successfully seems more appropriate than asserting
something we don't expect to always be true.
We've seen this assertion fail ~1k times in the past couple weeks, and that may be indicative
of other issues, but we can track that with the warning.

Test Plan: None

Reviewers: kajic

Reviewed By: kajic

Subscribers: changesbot, wwu

Differential Revision: https://tails.corp.dropbox.com/D99573

changes/listeners/mail.py

@@ -91,8 +91,11 @@ def send(self, msg, build):
                 'recipients': msg.recipients,
             }
         })
-        assert event, 'An email has already been sent for collection_id={} (build_id={}).'.format(
-            build.collection_id, build.id.hex)
+        # If we were unable to create the Event, we must've done so (and thus sent the mail) already.
+        if not event:
+            self.logger.warning('An email has already been sent for collection_id={} (build_id={}).'.format(
+                build.collection_id, build.id.hex))
+            return
 
         mail.send(msg)