Generic alpha log error message for failed ACL login (#6848)

* In case of an invalid login request, a generic error message will be printed in alpha logs

* used x.ErrorInvalidLogin per Ibrahim's suggestion

(cherry picked from commit 7088d59)

edgraph/access_ee.go

@@ -135,7 +135,7 @@ func (s *Server) authenticateLogin(ctx context.Context, request *api.LoginReques
 
 		if user == nil {
 			return nil, errors.Errorf("unable to authenticate through refresh token: "+
-				"user not found for id %v", userId)
+				"invalid username or password")
 		}
 
 		glog.Infof("Authenticated user %s through refresh token", userId)
@@ -152,10 +152,10 @@ func (s *Server) authenticateLogin(ctx context.Context, request *api.LoginReques
 
 	if user == nil {
 		return nil, errors.Errorf("unable to authenticate through password: "+
-			"user not found for id %v", request.Userid)
+			"invalid username or passowrd")
 	}
 	if !user.PasswordMatch {
-		return nil, errors.Errorf("password mismatch for user: %v", request.Userid)
+		return nil, x.ErrorInvalidLogin
 	}
 	return user, nil
 }