🎨 Optimise permissble function in user model

no issue

- if you destroy a user with an unknown user id, Ghost would crash
- because `userModel.hasRole` is undefined

- there is actually a bigger underlying architectual problem:
   - the permission check should rely on an existing user
   - so there should be a first api layer, which 1. validates (this code exists) and 2. ensures that requested database id's exist
   - but this requires a bigger refactoring

core/server/models/user.js

@@ -562,6 +562,12 @@ User = ghostBookshelf.Model.extend({
 
             // Get the actual user model
             return this.findOne({id: userModelOrId, status: 'all'}, {include: ['roles']}).then(function then(foundUserModel) {
+                if (!foundUserModel) {
+                    throw new errors.NotFoundError({
+                        message: i18n.t('errors.models.user.userNotFound')
+                    });
+                }
+
                 // Build up the original args but substitute with actual model
                 var newArgs = [foundUserModel].concat(origArgs);