gh-83371: Fix deadlock when a Pool callback raises an exception (GH-155777)

The exception killed the thread which handles results, so that the pool
hung forever.  It is now the result of the job and is raised by
AsyncResult.get(), with the original error as its context.

Co-authored-by: Sindri Guðmundsson <sindrigudmundsson@gmail.com>
Co-authored-by: Thomas Grainger <tagrain@gmail.com>