Fixes #6912/BZ 1126446 - Sync Plan enabled by default, 

checkbox to disable on edit
Whenever I save a sync plan that was created with an interval of 'none' before the update I get the following error from the server:  "An error occurred saving the Sync Plan: is not included in the list" which doesn't really explain to the user what happened or how they can fix it.

This diff includes the word interval into the error message which I think helps a lot:

```diff
diff --git a/engines/bastion/app/assets/javascripts/bastion/sync-plans/details/sync-plan-details-info.controller.js b/engines/bastion/app/assets/javascripts/bastion/sync-plans/details/sync-plan-details-info.controller.js
index 15c4120..a5a0823 100644
--- a/engines/bastion/app/assets/javascripts/bastion/sync-plans/details/sync-plan-details-info.controller.js
+++ b/engines/bastion/app/assets/javascripts/bastion/sync-plans/details/sync-plan-details-info.controller.js
@@ -45,7 +45,10 @@ angular.module('Bastion.sync-plans').controller('SyncPlanDetailsInfoController',
                     $scope.successMessages.push(translate('Sync Plan Saved'));
                 }, function (response) {
                     deferred.reject(response);
-                    angular.forEach(response.data.errors, function (errorMessage) {
+                    angular.forEach(response.data.errors, function (errorMessage, key) {
+                        if (angular.isString(key)) {
+                            errorMessage = [key, errorMessage].join(' ');
+                        }
                         $scope.errorMessages.push(translate("An error occurred saving the Sync Plan: ") + errorMessage);
                     });
                 });
```