Handle exit gracefully when running the script in interactive shell for debugging (i.e. in ipython)

ipython -i -c "%run ./opensubtitles-download.py --file movie.avi" was failing for me, this fixes that (and does not do anything bad, at least on my machine). I run the script for ipython since it is useful for further debugging - I got the namespaces and variables for free.

Github tells me I request two commits whereas I picked only one, strange:-/.