Fail gracefully when syncing back a missing draft.
Fail gracefully when syncing back a missing draft.

Summary:
This can happen if, for example, the draft is deleted before saving it
succeeds.

Test Plan: Stare at code.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D561

wakermahmud/sync-engine,Eagles2F/sync-engine,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,EthanBlackburn/sync-engine,jobscore/sync-engine,PriviPK/privipk-sync-engine,gale320/sync-engine,ErinCall/sync-engine,closeio/nylas,PriviPK/privipk-sync-engine,PriviPK/privipk-sync-engine,jobscore/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,closeio/nylas,jobscore/sync-engine,wakermahmud/sync-engine,nylas/sync-engine,EthanBlackburn/sync-engine,wakermahmud/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,PriviPK/privipk-sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,gale320/sync-engine,nylas/sync-engine,nylas/sync-engine,closeio/nylas,jobscore/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,gale320/sync-engine,EthanBlackburn/sync-engine,closeio/nylas