course imports should expect and handle attachment quota errors
course imports should expect and handle attachment quota errors

closes FOO-1394
flag=none

TEST PLAN:
  1) do course copies when over attachment quota
  2) errors work, but don't trigger sentry notifications

Change-Id: I69887d2472ea0050f2de346febb699753294dd56
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/255815
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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