wgengine/magicsock: log the peer failing disco writes are intended for

Updates tailscale/corp#31762

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -2026,7 +2026,7 @@ func (c *Conn) sendDiscoMessage(dst epAddr, dstKey key.NodePublic, dstDisco key.
 		// Can't send. (e.g. no IPv6 locally)
 	} else {
 		if !c.networkDown() && pmtuShouldLogDiscoTxErr(m, err) {
-			c.logf("magicsock: disco: failed to send %v to %v: %v", disco.MessageSummary(m), dst, err)
+			c.logf("magicsock: disco: failed to send %v to %v %s: %v", disco.MessageSummary(m), dst, dstKey.ShortString(), err)
 		}
 	}
 	return sent, err