Proposal: Decorate "overflow unpacking" errors with where in the unpacking the error occurred.

[EricIO]

I'm quite willing to do this work myself but wanted to first check if it is something that might get accepted.

We're seeing overflow unpacking uint16 errors in our system and are having trouble reproducing it locally / getting a packet capture to see what is going on. One thing that might help us pinpoint what is going on would be some better logging.

The proposal would be to decorate errors from the unpackUint* and related family of functions with where the error occurred. For example:

	hdr.Rrtype, off, err = unpackUint16(msg, off)
	if err != nil {
		return hdr, len(msg), msg, err
	}

to

	hdr.Rrtype, off, err = unpackUint16(msg, off)
	if err != nil {
		return hdr, len(msg), msg, fmt.Errorf("header.Rrtype: %w", err)
	}