try: add support for `finally'

This fixes #75

Signed-off-by: Julien Danjou <julien@danjou.info>
Enhance error handling

We now have a full backtrace if the compiler bugs, and if it's a user
problem we indicate him where he failed.

Signed-off-by: Julien Danjou <julien@danjou.info>
Merge branch 'jd/finally' of git://github.com/jd/hy into pr-94
removing the pass