WebSocket bad logic/error message

[nludban]

Long story short

With an aiohttp.ClientSession and ws_connect to an embedded data acquisition device, my application got a message of type aiohttp.MsgType.error:

Message(tp=<MsgType.error: 21>, data=WebSocketError('Received new fragment frame with non-zero opcode 0',), extra=None)

Which is generated in websocket.py parse_frame:

``'python
if fin == 0 and opcode == OPCODE_CONTINUATION and not continuation:
raise WebSocketError(
CLOSE_PROTOCOL_ERROR,
'Received new fragment frame with non-zero '
'opcode {!r}'.format(opcode))

## Expected behaviour

Provide a self-consistent error message.  Possibly correct some logic.  (It's very believable that the embedded device is actually sending bad frames.)

## Actual behaviour

Given that OPCODE_CONTINUATION = 0x0, the only message this block of code can deliver is that there's a non-zero opcode of constant value zero...

## Steps to reproduce

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## Your environment

Observed in aiohttp-0.21.6, this function does not appear to have any changes in the latest code.

[nludban]

Verified with tcpdump that the peer incorrectly inserted two octets of zero in the TCP stream.

Suggest replacing the error message "Received new fragment frame with non-zero opcode {!r}." with "Received continuation fragment as start of new frame."

[asvetlov]

Sorry, I think the value for proposed change is not high, new text is still now clean.
It means "something is broken in protocol structure", nothing more.

I believe the error is very rare, thus I want to do nothing with it.

[lock]

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