Untreated error callbacks when using websockets

[bvanelli]

There are two things:

• The parsing issue was caused by readline returning the actual error code (in this case 1001). I made it return b'' instead and EOF will be triggered in the next call
• __bool__ was returning false every time it was disconnected. I'm returning now if self._client exists instead.

If you want to test, simply use the script

import asyncio
import nats


async def main():
    async def cb(msg):
        print(msg)
    ws = await nats.connect("ws://localhost", error_cb=cb)
    await ws.subscribe("hello", cb=cb)
    for i in range(100):
        await ws.publish("hello", f"hello world {i}".encode())
        await asyncio.sleep(5)


if __name__ == '__main__':
    asyncio.run(main())

And turn off the connection to NATS. This will cause now EOF in both implementations.

Closes #361

[bvanelli]

@wallyqs do you have further points to address?