GUAC-1172: Upgrade angular-translate to 2.7.2. Configure required sanitization strategy. Use null instead of false for lack of login error.

guacamole/src/main/webapp/app/login/directives/login.js

@@ -77,7 +77,7 @@ angular.module('login').directive('guacLogin', [function guacLogin() {
          *
          * @type String
          */
-        $scope.loginError = false;
+        $scope.loginError = null;
 
         /**
          * All form values entered by the user, as parameter name/value pairs.