Replicate .NET behavior for exceptions in the finalizer thread.

Replicate .NET behavior for exceptions in the finalizer thread.

On .NET, when an exception occurs in the finalizer thread, it is
printed, the AppDomain.UnhandledException event is raised, and
the runtime is aborted (exact same thing as what happens when an
exception is thrown in a thread pool thread).

We now do the same.

Note that this is a breaking change; previously, we just silently
swallowed exceptions that occurred in the finalizer thread. Given
this, a test had to be adjusted (and renamed). An extra test has
been added to verify that the UnhandledException event is raised
correctly.