don't raise error in course export if external tool is missing

test plan:
* create a course with an external tool with a navigation tab
* permanently delete the tool in the console:

 ContextExternalTool.delete_all(:context_id => <INSERT COURSE ID HERE>)

* set a course setting (such as the tab configuration)
* copy the course
* should retain the course settings

fixes #CNVS-13661

Change-Id: Id3d810282e3eff3bc9cb0b98cdb96af73a503a28
Reviewed-on: https://gerrit.instructure.com/36531
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>