don't explode if web snapshotting fails

fixes CNVS-15449

test plan:
- make sure CutyCapt is enabled
- create a url submission assignment
- wait for the snapshot to happen
- go to speedgrader, the preview should show

Change-Id: Ib463c2a9c85361c2468332ef8f1e170c8641b03e
Reviewed-on: https://gerrit.instructure.com/40947
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>