Start trying to catch unsetting variables.

Unfortunately, I'm not yet sure how to catch the exception that's raised in the
deterministic3 test. It should raise a BridgeException saying "variable 'b' has
unset."
Fix test.

Fix suggested by Edd Barrett (and I'm an idiot for not thinking of it!).
Remove test accompanying previous commit.
Merge branch 'pypy_bridge_scopes' into pypy_bridge