stop processing new parallel importers when failed
stop processing new parallel importers when failed

fixes CORE-2200

test plan
 - cause an import to fail
 - no future parallel importers should process

Change-Id: I9d008434a39913037505e9cc4a64be299cfec0d1
Reviewed-on: https://gerrit.instructure.com/173645
Tested-by: Jenkins
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms