fix missing attachment error in content exports

test plan:
* create a course object with html content (such as a
 calendar event)
* create a link to a file, but corrupt the attachment id in the path
so it won't be found (e.g. /courses/x/files/y;
 set y to something nonexistent)
* exporting or copying the course should not prevent all objects
of that type from being exported (e.g. should still bring in
other calendar events)

fixes #CNVS-13929

Change-Id: I84b3958937021a2c2d07db4fb4cb689d067136ce
Reviewed-on: https://gerrit.instructure.com/37383
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>