Don't raise an assert inside an assert
Don't raise an assert inside an assert

Summary: Fixes T3341. Fetching `part.block.data` from S3 sometimes fail because of a networking/corrupted data error. I changed the code to use an intermediate variable, `part_data`, so that we don't raise an `AssertionError` inside of the exception handler.

Test Plan: Tested manually by raising an `AssertionError` inside the function.

Reviewers: spang, emfree

Reviewed By: emfree

Maniphest Tasks: T3341

Differential Revision: https://phab.nylas.com/D1895

closeio/nylas,Eagles2F/sync-engine,closeio/nylas,Eagles2F/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,gale320/sync-engine,ErinCall/sync-engine,nylas/sync-engine,nylas/sync-engine,closeio/nylas,closeio/nylas,ErinCall/sync-engine,jobscore/sync-engine,gale320/sync-engine,nylas/sync-engine,gale320/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,jobscore/sync-engine