Don't try to access `stack` of undefined exceptions.

Currently, throwing `undefined` will cause Jest to fatal: `TypeError: Cannot read property 'stack' of undefined`.
Actually, the current behavior of printing `Exception` is pretty weird, too. If a truthy value is thrown (including an empty object or the number 42), that value is printed. If the value is falsey (including undefined, null, or the number 0), then `Exception` is printed.

I think we can do better.