websocket: avoid WebSocketClosedError double/nested exception

[ploxiln]

Tracking the exception that was being handled when another exception
was raised is a new feature in Python-3, causing this:

Traceback (most recent call last):
  File "/...-packages/tornado/websocket.py", line 1104, in wrapper
    await fut
tornado.iostream.StreamClosedError: Stream is closed

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/...-packages/tornado/websocket.py", line 1106, in wrapper
    raise WebSocketClosedError()
tornado.websocket.WebSocketClosedError

This can be selectively avoided using the new Python-3 syntax:

raise WebSocketClosedException from None

I just find the double-exception to be more ugly than it has to be, a matter of taste I guess, so I humbly offer this easy change in case you find it acceptable.

[bdarnell]

As I said in #2824 (comment), I'm generally not a fan of from None - it loses information about where exactly in self._write_frame the exception was raised. Maybe that's rarely useful, but for the rare cases where it matters it seems worth the occasional doubled stack trace. (on the other hand, preventing stack traces from being logged at all for closed connections is generallly a good thing).