continue sis import if unique index error happens
continue sis import if unique index error happens

test plan
 - run enrollment import
 - it should work
 - if unique index error happens it should continue

Change-Id: If128dbe34b2e41a8139f5afb8d87f9d424dd024a
Reviewed-on: https://gerrit.instructure.com/99088
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

sfu/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,djbender/canvas-lms