Make SDExport.exit_gracefully more standalone

[redshiftzero]

Followup from #41 (see comment here)

I'm not sure how to best do this, but when I was working on #41, I was wondering if we can make SDExport.exit_gracefully more standalone to decouple more code from SDExport. It would be nice to extract that method into a utility function since it's used almost everywhere. The challenge is that part of its cleanup is (if it exists) it's removing a submission's tmpdir. We'd either need to pass the tmpdir around everywhere, which is messy, or just leave it as a method on SDExport, or do something else. If anyone has any ideas on how to nicely refactor this, get loud.

[rmol]

How about:

• dispensing with exit_gracefully and just letting exceptions bubble
• adding tmpdir to SDExport.__init__ arguments
• in entrypoint.start, using with tempfile.TemporaryDirectory() as tmpdirname: around construction/usage of SDExport, and catching any exceptions in one finally block where we ensure we print something and exit 0

It would mean some tedious refactoring of the exit_gracefully calls to raise exceptions from ExportStatus messages.