Cancelled __aexit__ handlers eat exceptions

This is what happens when you don't wrap your async context's exit handler in a shielded cancel scope:

… trick question. Nothing happens – the error gets dropped on the floor. Since this is a fairly common mistake (at least IME) I wonder whether we can do something about it. If not (which is what I suspect) we need to document this pitfall more loudly.

import trio

class bar:
    async def __aenter__(self):
        return self
    async def __aexit__(self, *tb):
        await trio.sleep(0)

async def foo():
    with trio.open_cancel_scope() as s:
        async with bar():
            s.cancel()
            raise RuntimeError("Duh")

trio.run(foo)

Start with the reproducer in the issue and review Trio's cancellation and async context-manager behavior. Determine whether the dropped exception is intended; if behavior cannot be changed safely, document the shielded cancel-scope requirement and make the expected outcome clear.