Added Thread::Pool.abort_on_exception attr_accessor.

Often, in tests, it is helpful for debugging if exceptions raised in worker threads get passed up to abort the application.  Failing silently is not helpful to the debugging process.  For regular threads, we use `Thread.abort_on_exception = true` for this purpose.  Again, we don't do this in production, but it is very helpful when testing and debugging.

In this PR, I added a Thread::Pool.abort_on_exception attr_accessor.

If true, `Task`s will allow raised exceptions to pass through.
(Similar to Thread.abort_on_exception)