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.

bin/tests/system/isctest/asyncserver.py

@@ -160,6 +160,7 @@ class AsyncServer:
             loop.run_until_complete(coroutine())
 
     async def _run(self) -> None:
+        self._setup_exception_handler()
         self._setup_signals()
         assert self._work_done
         await self._listen_udp()
@@ -177,9 +178,20 @@ class AsyncServer:
             loop = asyncio.get_event_loop()
         return loop
 
-    def _setup_signals(self) -> None:
+    def _setup_exception_handler(self) -> None:
         loop = self._get_asyncio_loop()
         self._work_done = loop.create_future()
+        loop.set_exception_handler(self._handle_exception)
+
+    def _handle_exception(
+        self, _: asyncio.AbstractEventLoop, context: Dict[str, Any]
+    ) -> None:
+        assert self._work_done
+        exception = context.get("exception", RuntimeError(context["message"]))
+        self._work_done.set_exception(exception)
+
+    def _setup_signals(self) -> None:
+        loop = self._get_asyncio_loop()
         loop.add_signal_handler(signal.SIGINT, functools.partial(self._signal_done))
         loop.add_signal_handler(signal.SIGTERM, functools.partial(self._signal_done))