Log instead of fatal for reindexing documents

Summary:
We have some issues with Elastic search (or maybe it's SMC) causing that indexing sporadically doesn't work.
Throwing in indexing stops the workflow and is annoying.
Not indexing doesn't have fatal consequences for the user and we can (and probably should) postpone it.

Test Plan: Thrown, looked at log.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3350