The Clownfish exception mechanism wraps the host language's exception
mechanism, making it possible to throw host exceptions from C code without
knowledge of the host language. We need to augment this capability
with a way to trap exceptions from C.
This new feature will enable us to clean up a couple places in the code base,
such as opening an index reader. More importantly, though, it will make it
possible to write tests in C which verify error handling.