turnitin: don't explode if submission_type has changed before job runs

fixes CNVS-16377

test plan:
- enable turnitin
- create an assignment that accepts uploads or urls
- first submit an upload that is turnitinable (like a word doc)
- then, quickly resubmit a URL
- the submit_to_turnitin job should not be listed in the failed jobs list

Change-Id: I2d4fd2f2f94e65c7f6086c338f6dbe8e64203c12
Reviewed-on: https://gerrit.instructure.com/43140
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>