improve moodle 2 handling of missing file references

test plan:
* import the packages referenced in the ticket
* should not fail catastrophically

fixes #CNVS-13226

Change-Id: Ib66ec7c7d5eea3331349f069db6035e592ab84c4
Reviewed-on: https://gerrit.instructure.com/35448
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>