[feature]: Improve error message when connecting peer with unknown address

[myxmaster]

When calling ConnectPeer (REST: POST /v1/peers) with a valid pubkey but no host:port, and the peer's address is not known via gossip, LND returns a generic EOF error.

A descriptive error message like no addresses known for peer <pubkey> would be helpful so that clients can distinguish this case from other connection failures and guide the user accordingly (e.g. "try providing host:port directly").