NextReader returns the next data message received from the peer. The returned messageType is either TextMessage or BinaryMessage.

There can be at most one open reader on a connection. NextReader discards the previous message if the application has not already consumed it.

Applications must break out of the application's read loop when this method returns a non-nil error value. Errors returned from this method are permanent. Once this method returns a non-nil error, all subsequent calls to this method return the same error.