How about we extend loop.stop() so that you can pass it an exception to
raise once the loop is stopped? This exception would then be thrown out of
run_forever(). There may be some delay (callbacks already scheduled will
run first) but it is how things were meant to be.

FWIW this isn't really enough to ensure cleanup happens before destructors
run -- when the loop exits, tasks may still be active unless you keep track
of all of them and explicitly cancel them (and run the loop until they have
processed the cancellation).