ignore errors on individual shards when scrubbing messages

Change-Id: I9745053f78503157ae19ddfcb8005958cc753acf
Reviewed-on: https://gerrit.instructure.com/35451
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>