Passes through asyncio.CancelledError in tornado.gen.coroutine

[AshwinDeshpandeBG]

If a tornado.coroutine raises an asyncio.CancelledError, it is handled badly as an asyncio.CancelledError is not an Exception, but there is no special logic in the tornado.coroutine to handle this BaseException. We suspect this bug was created in python 3.8 when asyncio.CancelledError changed from an Exception to a BaseException.

See bug report: #3537

[bdarnell]

Hmm, is f.set_exception(CancelledError) equivalent to f.cancel()? I imagine they're slightly different because f.cancelled() would return false, but I'm not sure if it matters. We might need a separate branch in the try/except for CancelledError.

Or maybe we should be catching all errors (bare except: or except BaseException). That's what we do in some other places (see #3581) and I think the same logic might apply here.