Implement catchable WebsocketClosed

See #2220

In v21.9, a new WebsocketClosed exception was added.

While you can to do the following:

@app.websocker("/")
async def handler(request, ws):
    try:
        ...
    except asyncio.CancelledError:
        print("connection closed")

We would like to add support for this pattern:

@app.websocker("/")
async def handler(request, ws):
    try:
        ...
    except sanic.exceptions.WebsocketClosed:
        print("connection closed")

[ashleysommer]

@ahopkins I think there might’ve been some miscommunication around what the WebsocketClosed exception is for.

In #2248, WebsocketsClosed exception was added as a replacement for throwing sanic.ServerError exceptions if a user tries to read or write to a websocket connection that is closed or is closing. This also prevented the KeepAlive-ping background task from emitting a ServerError needlessly when the websocket connection was closed.

In theory (if the timing it right) it is possible to catch it like this:

@app.websocker("/")
async def handler(request, ws):
    try:
        while(True):
            ws.write("hello") # <-- this could throw WebsocketClosed when the socket closes
    except sanic.exceptions.WebsocketClosed:
        print("connection closed")

However also in #2248, I introduced more capability for the websockets connection to raise asyncio.CancelledError into the handler, to give the handler early warning that the connection is closing, and prevent the handler from continuing to make send() and recv() calls. So this change significantly decreases the possibility of the handler seeing a WebsocketClosed exception, because it would see asyncio.CancelledError first, and break from its handling.

If we really want the ability to catch a websocket-specific exception, rather than asyncio.CancelledError, then I suggest we create a new exception like WebsocketCancelled that subclasses asyncio.CancelledError, and use that instead.

Then the handler could use code like this to cover both:

@app.websocker("/")
async def handler(request, ws):
    try:
        ...
    except (WebsocketClosed, WebsocketCancelled)
        print("connection closed")

[ahopkins]

May be impacted by: https://github.com/sanic-org/sanic/pull/2513