watchtower/wtclient/interface: add LoadTower and mock impl

watchtower/wtdb/tower.go

@@ -1,6 +1,7 @@
 package wtdb
 
 import (
+	"errors"
 	"net"
 	"sync"
 
@@ -8,6 +9,12 @@ import (
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
+var (
+	// ErrTowerNotFound signals that the target tower was not found in the
+	// database.
+	ErrTowerNotFound = errors.New("tower not found")
+)
+
 // Tower holds the necessary components required to connect to a remote tower.
 // Communication is handled by brontide, and requires both a public key and an
 // address.