Missing failure on wrong git submodule SHA1

When scheduling a CI early warning run on a change with a wrong submodule SHA1, the run just hangs in "Waiting for sources", and doesn't give any useful feedback.

The underlying git archive call for the submodule fails (correctly), but the error is not handled gracefully or propagated back from the source storage service to the work item.

Successful fixing of this task involves introducing error handling for this case.