These return an untyped protected property, so we can't rely on
the type being correct.
Also add return types to the interface -- normally this would be
a no-go, but Throwable is a special interface that can only
be implemented internally, so we control all implementations.
We also change `Generator::throw()` to expect a `Throwable` in the
first place, and we now throw a TypeError instead of returning `false`
from `Exception::getTraceAsString()`.
