Add logging when assertion fails in blob saving.
Add logging when assertion fails in blob saving.

Summary:
This is happening on prod, bafflingly. Add some logging so we can see what's
up.

Test Plan: None.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D723

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