Add a missing import

server/server.go

@@ -3,6 +3,7 @@ package server
 import (
 	"crypto/tls"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"net/http"
@@ -182,10 +183,6 @@ func New(service *as.AccountService, backend as.Backend, leaderAddr string, clie
 
 var emptyResponse struct{}
 
-type jsonError interface {
-	JSON() []byte
-}
-
 // A forwardServer is just a fancy httputil.ReverseProxy with loop
 // detection (we don't want to receive proxied requests if we are not
 // the leader).