fix handling of redis del during blacklist of single node in ring

test plan:
 * configure a ring of redis cache servers
 * make sure at least one is not accessibly
 * login to canvas
 * it should not error

Change-Id: I46c464649438225f080f7097dbfc996260aea6cd
Reviewed-on: https://gerrit.instructure.com/28470
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>