[IPv6] Verify proper error handling of error condition caused by packet with Multicast Destination Address - UDP Port unreachable (cfr. Tahi 5.15)

[Q-Thomas]

If an UDP packet is received, that was destined to an IPv6 multicast address with a port that is not being used on the NUT (e.g. 9000), IPv6 should NOT send an ICMPv6 error message.

There are however two exceptions to this rule:

(1) the Packet Too Big Message - Section 3.2 - to allow Path MTU discovery to work for IPv6 multicast
(2) the Parameter Problem Message, Code 2 - Section 3.4 - reporting an unrecognized IPv6 option that has the Option Type highest-order two bits set to 10).