Fix hang when Websocket Read is interrupted

Updated to catch the quit signal and break out of the loop. Tested that ^C shutsdown `btcwallet` as expected, with the same env in which it was hanging earlier.
I just pushed a btcrpclient fix that causes requests to error when the client has already shutdown, so this check shouldn't be necessary (and isn't really right to begin with, a disconnected client could reconnect and the request would then be reissued).