Do not generate error messages when we receive our own gossip

When a peer sends us the routing graph, it may include gossip
messages for our channels, despite it not being a party to them.
This is completely fine, but we currently print a somewhat-scary
looking log messages in these cases, eg:

```
ERROR [lightning::ln::channelmanager:4104] Got a message for a channel from the wrong node!
TRACE [lightning::ln::peer_handler:1267] Handling SendErrorMessage HandleError event in peer_handler for node ... with message Got a message for a channel from the wrong node!
```

Instead, we should simply not consider this an "error" condition
and stay silent.