Roll back transaction when handling DebusineAPIException

rest_framework.views.exception_handler rolls back the transaction when handling APIException, but our specialized handling of DebusineAPIException failed to do so. This probably caused various issues in our API views, but most notably it meant that we would create workflows despite immediately-detectable validation errors.

I audited all our API views for patterns that would indicate problems resulting from this change. The only one I found was also found by the test suite: UploadFileView.put needs to delete the FileUpload row on a hash mismatch, and that needs to be committed despite the exception. I added a rollback_transaction argument to DebusineAPIException to provide a simple way to control this behaviour.

Fixes: #432 (closed)