[slozier]

Fix remaining test_contextlib tests.

throw() raised StopIteration('') whenever the generator finished, so:

• a return value was lost
• a StopIteration raised in the body wasn't propagated as-is (and so wasn't turned into RuntimeError by generator_stop).

For example (these are covered by the test_contextlib and test_yield_from):

def g():
    try: yield 1
    except ValueError: return 5

gi = g()
next(gi)
try:
    gi.throw(ValueError)
except StopIteration as e:
    assert e.value == 5

def g():
    yield

stop = StopIteration()
gi = g()
next(gi)
try:
    gi.throw(stop)
except StopIteration as e:
    assert e is stop

Part of #1