Ensure uncaught exceptions kill custom servers

Ensure uncaught exceptions kill custom servers

Uncaught exceptions raised by tasks running on event loops are not
handled by Python's default exception handler, so they do not cause
scripts to die immediately with a non-zero exit code.  Set up an
exception handler for AsyncServer code that makes any uncaught exception
the result of the Future that the top-level coroutine awaits.  This
ensures that any uncaught exceptions cause scripts based on AsyncServer
to immediately exit with an error, enabling the system test framework to
fail tests in which custom servers encounter unforeseen problems.

(cherry picked from commit ec4c92d9d59d1ba0ee3242485965afbaeb62c847)