[Extractor] Add Finding Win 7z Path via Registry

It should be try..except..else. Example from existing code: https://github.com/deluge-torrent/deluge/blob/develop/deluge/ui/gtkui/common.py#L163

One major reason is that it makes it easier to understand in the future when looking at the code what was actually being guarded.  http://www.jeffknupp.com/blog/2013/02/06/write-cleaner-python-use-exceptions/

Also could you keep the existing list of win_7x_exes and insert the path from reg at `index 1` in the `else`.  It doesn't hurt to have a fallback to default location.