don't count ignored redis errors in stats

refs CNVS-15570

test plan: trigger an ignored redis error, such as running
`Canvas.redis.renamenx('no-such-key', 'new-key')` in console, and verify
that rails doesn't log a 'Failure handling redis' message, or send
anything to statsd.

Change-Id: Ie4480fcf053d626ba8dbdf26672b18815267fa86
Reviewed-on: https://gerrit.instructure.com/41805
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>