🐛 Fixed error when deleting non-existent snippet

fixes https://github.com/TryGhost/Team/issues/809

- Bookshelf won't throw a `NotFoundError` unless `require=true` in the
  options
- this is present in most other API endpoints, so it's just simply
  missing from the snippet one
- without this, Ghost will crash with a 500 saying `Cannot read property
  'destroy' of null`
- this commit adds `require=true` to the destroy options for both the canary +
  v3 endpoints

core/server/api/canary/snippets.js

@@ -97,6 +97,8 @@ module.exports = {
         },
         permissions: true,
         query(frame) {
+            frame.options.require = true;
+
             return models.Snippet.destroy(frame.options)
                 .then(() => null)
                 .catch(models.Snippet.NotFoundError, () => {

core/server/api/v3/snippets.js

@@ -97,6 +97,8 @@ module.exports = {
         },
         permissions: true,
         query(frame) {
+            frame.options.require = true;
+
             return models.Snippet.destroy(frame.options)
                 .then(() => null)
                 .catch(models.Snippet.NotFoundError, () => {