handle_error(ReqId, Reason, UserData) -> void()
This function is called when the manager needs to communicate an "asynchronous" error to the user: e.g. failure to send an asynchronous message (i.e. encoding error), a received message was discarded due to security error, the manager failed to generate a response message to a received inform-request, or when receiving an unexpected PDU from an agent (could be an expired async request).
If ReqId is less then 0, it means that this information was not available to the manager (that info was never retrieved before the message was discarded).
For SnmpInfo see handle_agent below.
Note that there is a special case when the value of ReqId has the value of the atom netif. This means that the NetIF process has suffered a "fatal" error and been restarted. With possible loss of traffic!