don't raise/record an error when a file link can't be found
don't raise/record an error when a file link can't be found

refs sentry error 33094

This isn't an actionable error, it typically just means bad content was
imported. It's also extremely common due to content imports. Note that
the import warning "Missing links found in imported content" is still
recorded and displayed in the imports UI, this change just gets rid of
the error report.

test plan: import a common cartridge package that includes an assessment
question that has html pointing to a non-existent image file. no error
report should be generated, though you'll still see a warning in the ui
about a missing link.

Change-Id: Ic44ee446ed648388c87e022b80c695816f890e23
Reviewed-on: https://gerrit.instructure.com/71162
Tested-by: Jenkins
Reviewed-by: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

roxolan/canvas-lms,instructure/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,HotChalk/canvas-lms,fronteerio/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,dgynn/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,dgynn/canvas-lms,HotChalk/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,sfu/canvas-lms