Openbazaar python launcher

new pull request after 1st round of reviews.
Thanks @Renelvon this has the merged changes from your repo.

(@Renelvon I kept that REVERT commit you put there as I'm not sure I understood what was right or wrong about it, I figured it'd be easier to fix here now)
If you intend to re-`raise` the current exception, you should just write `raise`, otherwise Python will believe there is a *new* exception raised and the stacktrace will look confusing.