rescue system errors like Errno::ETIMEDOUT in redis

turns out the redis client doesn't always translate these into
Redis::BaseConnectionError -- specifically, we're seeing
Errno::ETIMEDOUT on writes

Change-Id: Ifa040f2768a1035a8a7e7557cd5642928e025bd3
Reviewed-on: https://gerrit.instructure.com/39899
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>