Fix KeyError on message deduplication.

Summary:
If we don't have X-GM-MSGID for a given UID, it means that the UID
appeared in the folder between the download of remote_g_metadata and the
fetching of the remote UID list. This race happens rarely, and the
easiest solution is to assume we have to download the full message in
this case. If we actually do already have a message with this
X-GM-MSGID, we will deduplicate it upon database insersion.

Test Plan:
Running it on staging for a while. The patch is small enough that
I think we can jump straight to that.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D165