rpcserver: return consistent grpc NotFound error for GetChaninfo

So that we can properly convert this NotFound error back to our
`graphdb.ErrEdgeNotFound` error if we are making this call to another
node via RPC.

rpcserver.go

@@ -6989,7 +6989,10 @@ func (r *rpcServer) GetChanInfo(_ context.Context,
 	default:
 		return nil, fmt.Errorf("specify either chan_id or chan_point")
 	}
-	if err != nil {
+	switch {
+	case errors.Is(err, graphdb.ErrEdgeNotFound):
+		return nil, status.Error(codes.NotFound, err.Error())
+	case err != nil:
 		return nil, err
 	}