client/network-gossip/state_machine: Fix log message (#5350)

A gossip validator returns `ValidationResult::Discard` for messages that
should not be processed nor kept due to not being useful for the node.
Examples are out-of-scope messages or messages with bad signatures. The
corresponding log message did not reflect this, thus the update through
this patch.

One caveat is a finality grandpa neighbor packet. Given that the concept
of neighbor packets is handled in the finality grandpa gossip validator
only, valid neighbor packets are always discarded on the
`client/network-gossip` layer.