gossipd: downgrade broken message that can actually happen.

I saw this in test_bech32_funding: the peer disconnected and told gossipd before lightningd
relayed a local_channel_announcement from the subd:

```
lightningd-1 2022-07-14T08:46:32.352Z DEBUG   020ba48216be53051ba8c661c641b5d9c3547c44bfcc43bf4d8362f0dfce0e950d-channeld-chan#1: billboard: Funding transaction locked. Channel announced.
lightningd-1 2022-07-14T08:46:33.353Z DEBUG   connectd: drain_peer
lightningd-1 2022-07-14T08:46:33.353Z DEBUG   connectd: drain_peer draining subd!
lightningd-1 2022-07-14T08:46:33.353Z DEBUG   020ba48216be53051ba8c661c641b5d9c3547c44bfcc43bf4d8362f0dfce0e950d-lightningd: peer_disconnect_done
lightningd-1 2022-07-14T08:46:33.354Z INFO    020ba48216be53051ba8c661c641b5d9c3547c44bfcc43bf4d8362f0dfce0e950d-channeld-chan#1: Peer connection lost
lightningd-1 2022-07-14T08:46:33.354Z INFO    020ba48216be53051ba8c661c641b5d9c3547c44bfcc43bf4d8362f0dfce0e950d-chan#1: Peer transient failure in CHANNELD_NORMAL: channeld: Owning subdaemon channeld died (62208)
lightningd-1 2022-07-14T08:46:33.354Z DEBUG   connectd: maybe_free_peer freeing peer!
lightningd-1 2022-07-14T08:46:33.354Z DEBUG   0228af54fd951097caa2ceea5546a37bcc7d7f746e1cb7cb549e3edcd1797a1d80-hsmd: Got WIRE_HSMD_CUPDATE_SIG_REQ
lightningd-1 2022-07-14T08:46:33.354Z DEBUG   plugin-funder: Cleaning up inflights for peer id 020ba48216be53051ba8c661c641b5d9c3547c44bfcc43bf4d8362f0dfce0e950d
lightningd-1 2022-07-14T08:46:33.354Z **BROKEN** gossipd: Unknown peer 020ba48216be53051ba8c661c641b5d9c3547c44bfcc43bf4d8362f0dfce0e950d for local_channel_announcement
```

Signed-off-by: Rusty Russell <[email protected]>

gossipd/gossipd.c

@@ -300,8 +300,8 @@ static void handle_local_channel_announcement(struct daemon *daemon, const u8 *m
 	/* We treat it OK even if peer has disconnected since (unlikely though!) */
 	peer = find_peer(daemon, &id);
 	if (!peer)
-		status_broken("Unknown peer %s for local_channel_announcement",
-			      type_to_string(tmpctx, struct node_id, &id));
+		status_debug("Unknown peer %s for local_channel_announcement",
+			     type_to_string(tmpctx, struct node_id, &id));
 
 	err = handle_channel_announcement_msg(daemon, peer, cannouncement);
 	if (err) {