_exceptions.py: Modify BstError API to allow optional retry

job.py: Changes to the logic surrounding retry attempts and child process return codes
element.py, source.py: ElementError and SourceError also implement this change.

These exceptions now have an optional parameter of temporary which defaults to false. This will potentially break
backwards compatibility where exceptions were previously raised and a retry was intended.

To trigger a retry, one must now raise their SourceError or ElementError with temporary=True.

This aims to fix #397.