Add error handling when the ChannelType is not known

While breaking engine-core I found out that engine returns None when the ChannelType is not known. I would suggest to refactor this together with #145 by raising an exception.

        if channel_name in ChannelType.QUEUE.channels:
            self.logger.debug(f"Create new QUEUE channel '{channel_name}'")
            return QueueChannel(channel_index, channel_name, mixer)
        if channel_name in ChannelType.HTTP.channels:
            self.logger.debug(f"Create new STREAM channel '{channel_name}'")
            return StreamChannel(channel_index, channel_name, mixer)
        if channel_name in ChannelType.LIVE.channels:
            self.logger.debug(f"Create new LINE channel '{channel_name}'")
            return LineChannel(channel_index, channel_name, mixer)