ot::LinkQualityInfo::GetMessageErrorRate() const method

Returns the message error rate for the link. The rate is maintained over a window of (roughly) last `OPENTHREAD_CONFIG_IPV6_TX_ERR_RATE_AVERAGING_WINDOW` (IPv6) messages. Note that a larger (IPv6) message can be fragmented and sent as multiple MAC frames. The message transmission is considered a failure, if any of its fragments fail after all MAC retry attempts.